Light modulation apparatus, image pickup apparatus, and drive methods therefor

ABSTRACT

Disclosed are a light modulation apparatus including a liquid crystal device; and a polarizing plate disposed in an optical path of light made incident on said liquid crystal device; wherein said liquid crystal device is of a guest-host type using a negative type liquid crystal as a host material, an image pickup apparatus using the light modulation apparatus, and methods of driving the light modulation apparatus and image pickup apparatus.

The present application is a divisional of U.S. patent application Ser.No. 09/711,651 filed Nov. 13, 2000 now U.S. Pat. No. 6,804,037, whichclaims priority to Japanese Application Nos. P11-322184 filed Nov. 12,1999, P11-322185 filed Nov. 12, 1999, P11-322186 filed Nov. 12, 1999,P11-322187 filed Nov. 12, 1999 and P2000-319879 filed Oct. 19, 2000, allof which are hereby incorporated by reference to the extent permitted bylaw.

BACKGROUND OF THE INVENTION

The present invention relates to a light modulation apparatus formodulating the quantity of incident light and outputting the modulatedlight and an image pickup apparatus using the light modulationapparatus, and methods of driving the light modulation apparatus and theimage pickup apparatus.

Light modulation apparatuses have been known of a type including aliquid crustal cell, typically, a twisted nematic (TN) type liquidcrystal cell or a guest host type liquid crystal cell (GH cell), and apolarizing plate.

FIGS. 1A and 1B are schematic views showing an operational principal ofa related art light modulation apparatus mainly including a polarizingplate 1 and a GH cell 2, and FIG. 1C is a graph showing a rectangularwaveform of a drive voltage to be applied to the GH cell 2. In thefigures, for ease of description, components of a liquid crystal deviceother than the GH cell 2, for example, two glass substrates betweenwhich the GH cell 2 is held, operational electrodes, and liquid crystalalignment films formed on the substrates are omitted. The GH cell 2contains liquid crystal molecules 3 and dichroic dye molecules 4. Thedichroic dye molecules 4 have a positive type (p-type) light absorptionanisotropy capable of absorbing light in the alignment direction ofmajor axes of the molecules, and the liquid crystal molecules 3 have apositive type (p-type) dielectric constant anisotropy.

FIG. 1A shows a state of the GH cell 2 when no voltage is appliedthereto. Incident light 5, which passes through the polarizing plate 1,is linearly polarized by the polarizing plate 1. In this related artlight modulation apparatus, since the polarization direction of thelinearly polarized light corresponds to the alignment direction of themajor axes of the dichroic dye molecules 4, the light is absorbed in thedichroic dye molecules 4, with a result that the transmittance of the GHcell 2 is reduced.

When a voltage having a rectangular waveform shown in FIG. 1C is appliedto the GH cell 2 as shown in FIG. 1B, the alignment direction of themajor axes of the dichroic dye molecules 4 becomes perpendicular to thepolarization direction of the linearly polarized light, with a resultthat the light is little absorbed in the GH cell 2, that is, most of thelight passes through the GH cell 2.

In the case of using a GH cell including a negative type (n-type)dichroic dye molecules capable of absorbing light in the alignmentdirection of minor axes of the molecules, the relationship between lightabsorption and light transmission of the GH cell is reversed to that ofthe GH cell 2 including the positive type dichroic dye molecules 4. Tobe more specific, the light is not absorbed in the GH cell including thenegative type dichroic dye molecules when no voltage is applied thereto,and light is absorbed in the GH cell including the negative typedichroic dye molecules when a voltage is applied thereto.

An optical density (absorbance) ratio of the light modulation apparatusshown in FIGS. 1A to 1C, that is, a ratio of an optical density of theapparatus upon application of a voltage to an optical density thereofupon application of no voltage is about 10. This optical density ratioof the apparatus shown in the figures is as large as about twice anoptical density ratio of a light modulation apparatus including only theGH cell 2 without use of the polarizing plate 1.

The related art light modulation apparatus shown in the figures has aproblem. Since the polarizing plate 1 is fixed in an effective opticalpath of light, part of light, for example, 50% of light is usuallyabsorbed in the polarizing plate 1, and further light may be reflectedfrom the surface of the polarizing plate 1. As a result, the maximumtransmittance of light passing through the polarizing plate 1 cannotexceed a certain value, for example, 50%, and accordingly, the quantityof light passing through the light modulation apparatus is significantlyreduced by light absorption of the polarizing plate 1. This problem isone of factors which make it difficult to put a light modulationapparatus using a liquid crystal cell into practical use.

On the other hand, various kinds of light modulation apparatuses usingno polarizing plate have been proposed. Examples of these apparatusesinclude a type using a stack of two GH cells in which the GH cell at thefirst layer absorbs a polarization component in the direction identicalto that of polarized light and the GH cell at the second layer absorbs apolarization component in the direction perpendicular to the polarizedlight; a type making use of a phase transition between a cholestericphase and a nematic phase of a liquid crystal cell; and a high polymerscattering type making use of scattering of liquid crystal.

These light modulation apparatuses using no polarizing plate have aproblem. Since the optical density (absorbance) ratio between uponapplication of no voltage and upon application of a voltage is, asdescribed above, as small as only 5, the contrast ratio of the apparatusis to small to normally carry out modulation of light at any location ina wide range from a bright location to a dark location. The lightmodulation apparatus of the high polymer scattering type has anotherproblem in significantly degrading, when the apparatus is used for animage pickup apparatus, the image formation performance of an opticalsystem of the image pickup apparatus.

The related art light modulation apparatus presents a further problem.Since the transmittance in a transparent state may become dark dependingon the kind of a liquid crystal device used for the apparatus, if animage pickup apparatus provided with the light modulation apparatus isintended to pickup image with a sufficient light quantity in such atransparent state, the light modulation apparatus is required to beremoved from an optical system of the image pickup apparatus.

The related art light modulation apparatus has the following problemassociated with the drive thereof. To drive the related art lightmodulation apparatus, the transmittance has been controlled bymodulating a magnitude of a DC voltage or AC voltage applied to theapparatus; however, for the light modulation apparatus at a consumerlevel, it is difficult to accurately perform voltage control and toobtain a characteristic having a low threshold value; a limitation liesin the number of gradation of the transmittance level; and D/Aconversion is required for voltage control based on the intensity oftransmission light, to raise a circuit cost.

The drive of the related art light modulation apparatus, particularly,of a type including a negative type liquid crystal having a negativedielectric constant anisotropy has another problem. In the related artlight modulation apparatus, the transmittance has been changed with alarge step from a current transmittance into a target transmittance;however, upon such a change in transmittance with a large step,particularly, from a transmittance in a transparent state into atransmittance in a light shield state, there occurs a defect inalignment of liquid crystal molecules, resulting in unstable opticalcharacteristics, for example, in-plane non-uniformity in transmittance(which will be described later).

To be more specific, when a voltage applied to the liquid crystal ischanged with a large step for changing the transmittance with a largestep, there occurs a transient state in which liquid crystal moleculesare aligned in different directions, and if such a transient statecontinues for a time being long enough to exert an effect on thetransmittance, there appears in-plane non-uniformity in transmittance.In general, the transient state disappears after an elapse of a certaintime required for re-alignment of liquid crystal molecules and pigmentmolecules; however, in the worst case, the transient state may partiallyremain even after an elapse of a long time.

A further problem of the drive of the related art light modulationapparatus is that even in a state in which drive pulses with a specificcontrol waveform are applied to a liquid crystal device of the lightmodulation apparatus, there occurs a variation in transmittance due to achange in temperature of the environment in which the apparatus isdisposed.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a light modulationapparatus capable of improving the transmittance, enhancing the contrastratio, and keeping constant the quantity of light.

To achieve the first object, according to a first invention, there areprovided a light modulation apparatus including a liquid crystal deviceand a polarizing plate disposed in an optical path of light madeincident on the liquid crystal device, wherein the liquid crystal deviceis a guest-host type liquid crystal device using a negative type liquidcrystal as a host material, and an image pickup apparatus including thelight modulation apparatus disposed in an optical path of an opticalsystem of the image pickup system.

The negative type liquid crystal of the liquid crystal device may have anegative dielectric constant anisotropy, and the guest material may be apositive type or negative type dichroic dye molecular material.

With the above configurations of the first invention, a negative typeliquid crystal having a negative type dielectric constant anisotropy(Δ∈) is used as the host material constituting part of the liquidcrystal device disposed on the optical path, and accordingly, thetransmittance upon light transmission, particularly, in a transparentstate can be largely improved as compared with a light modulation deviceincluding a liquid crystal device using a positive type liquid crystal(Δ∈>0), and thereby the light modulation apparatus can be fixedlypositioned in an optical system of the image pickup apparatus.

The light modulation apparatus of the first invention, in which thepolarizing plate is disposed in the optical path of light made incidenton the above-described unique liquid crystal device, is furtheradvantageous in that an optical density (absorbance) ratio of theapparatus between upon application of no voltage and upon application ofa voltage is improved, to increase the contrast ratio of the apparatus,thereby normally carrying out modulation of light at any location in awide range from a bright location to a dark location.

A second object of the present invention is to provide a lightmodulation apparatus capable of easily, accurately controlling thetransmittance, reducing a threshold value, improving the number ofgradation, simplifying a drive circuit, and lowering the cost, an imagepickup apparatus using the light modulation apparatus, and methods ofdriving the light modulation apparatus and the image pickup apparatus.

To achieve the second object, according to a second invention, there areprovided a light modulation apparatus including a liquid crystal device,a drive pulse generation unit for driving the liquid crystal device, anda pulse width control unit for modulating a pulse width of each drivepulse thereby controlling the transmittance of light made incident onthe liquid crystal device, and an image pickup apparatus including thelight modulation apparatus disposed in an optical path of an opticalsystem of the image pickup apparatus.

The pulse width of each drive pulse may be modulated with its pulseheight kept constant. An average per unit time of positive and negativepulse heights of drive pulses applied between drive electrodes of theliquid crystal device upon modulation of the pulse width of each drivepulse may be preferably nearly zero for eliminating a bias action due toa DC component which is one of causes of flicker.

The modulation of the pulse width of each drive pulse may be performedin such a manner that the waveform of each drive pulse is present in aperiod of a basic frequency. The basic frequency and the modulated pulsewidth may be adjusted in such a manner as to prevent the occurrence offlicker in stationary drive of the light modulation apparatus. The lightmodulation apparatus may further include a drive circuit unit, and eachdrive pulse whose waveform is present in the period of the basicfrequency may be generated in synchronization with a clock generated bythe drive circuit unit.

The light modulation apparatus may further include a control circuitunit, and may be configured such that luminance information of the lightemerged from the liquid crystal device is fed back to the controlcircuit unit, and the pulse width of each drive pulse is modulated insynchronization with a clock generated by the drive circuit unit on thebasis of a control signal supplied from the control circuit unit. Theimage pickup apparatus including the light modulation apparatus mayfurther include an image pickup device disposed on the light outgoingside of the light modulation apparatus, and may be configured such thatthe drive circuit unit is provided in the image pickup device, and anoutput signal from the image pickup device is fed back as luminanceinformation to the control circuit unit of the light modulationapparatus and the pulse width of each drive pulse is modulated insynchronization with a clock generated by the drive circuit unit on thebasis of a control signal supplied from the control circuit unit.

With the above configurations of the second invention, the transmittanceis controlled by modulating the pulse width of each drive pulse appliedto the liquid crystal device for light modulation, and accordingly, ascompared with control of the transmittance by modulating the magnitudeof a voltage, the transmittance can be easily, accurately controlledbecause the pulse width can be easily, accurately modulated insynchronization with a clock generated by the pulse width control unit;the change in transmittance by modulation of the pulse width is allowedto occur at a low threshold value; the transmittance can be easily,accurately controlled because the change in transmittance by modulationof the pulse width is relatively moderate; the number of gradation canbe increased; and the need of D/A conversion can be eliminated tothereby reduce a circuit cost.

In particular, for a light modulation apparatus at a consumer level, themodulation of the pulse width of each drive pulse is advantageous interms of its accuracy and easiness, and more particularly, in the caseof mounting the light modulation apparatus in a recent digital controltype equipment, the control of the pulse width on the time axis can beexpected to realize a highly accurate control system of the equipment ata low cost.

To achieve the second object, according to the second invention, thereare also provided methods of driving a light modulation apparatus and animage pickup apparatus, each method including the step of driving aliquid crystal device by modulating the pulse width of each drive pulseapplied to the liquid crystal device thereby controlling thetransmittance of light made incident on the liquid crystal device.

The methods of driving the light modulation apparatus and image pickupapparatus according to the second invention are each advantageous indriving the light modulation apparatus and image pickup apparatus with agood controllability.

In this way, according to the second invention, it is very importantthat the unique means of modulating the pulse width of each drive pulse,whose waveform is selected for improving and stabilizing the opticalcharacteristics of the liquid crystal device of the light modulationapparatus, is used for the drive of the liquid crystal device of thelight modulation apparatus.

A third object of the present invention is to provide a light modulationapparatus capable of stably controlling the transmittance withoutoccurrence of a defect in alignment of liquid crystal molecules, animage pickup apparatus using the light modulation apparatus, and methodsof driving the light modulation apparatus and the image pickupapparatus.

To achieve the third object, according to a third invention, there areprovided a light modulation apparatus including a liquid crystal device,and a pulse control unit for changing the transmittance of light madeincident on the liquid crystal device from a current transmittance intoa target transmittance by applying drive pulses controlled with at leasttwo-steps to the liquid crystal device, and an image pickup apparatusincluding the light modulation apparatus disposed in an optical path ofan optical system of the image pickup apparatus.

The pulse height or pulse width of each drive pulse may be controlledwith at least two-steps.

The light modulation apparatus may further include a drive circuit unit,and may be configured such that the drive pulse may be generated insynchronization with a clock generated by the drive circuit unit.

The light modulation apparatus may further include a control circuitunit, and may be configured such that luminance information of the lightemerged from the liquid crystal device is fed back to the controlcircuit unit, and each drive pulse is generated in synchronization witha clock generated by the drive circuit unit on the basis of a controlsignal supplied from the control circuit unit. The image pickupapparatus including the light modulation apparatus may further includean image pickup device disposed on the light outgoing side of the lightmodulation apparatus, and may be configured such that the drive circuitunit is provided in the image pickup device, and an output signal fromthe image pickup device is fed back as luminance information to thecontrol circuit unit of the light modulation apparatus and each drivepulse is generated in synchronization with a clock generated by thedrive circuit unit on the basis of a control signal supplied from thecontrol circuit unit.

With the above configurations of the third invention, the drive pulsesto be applied to the liquid crystal device for light modulation arecontrolled with at least two-steps (from a low voltage to a highvoltage), and accordingly, as compared with the related art lightmodulation apparatus in which the voltage is steeply changed, thetransmittance can be controlled to be uniform over the entire plane ofthe liquid crystal device by applying a preparation pulse, whose heightis low enough to prevent occurrence of a defect in alignment of liquidcrystal molecules, thereby tilting the liquid crystal molecules to someextent, and then applying a final pulse required for achieving a desiredtransmittance.

To achieve the third object, according to the third invention, there arealso provide methods of driving a light modulation apparatus and animage pickup apparatus, each method including the step of changing thetransmittance of light made incident on a liquid crystal device from acurrent transmittance into a target transmittance by applying drivepulses controlled with at least two-steps to the liquid crystal device.

The methods of driving the light modulation apparatus and image pickupapparatus according to the third invention are each advantageous indriving the light modulation apparatus and image pickup apparatus with agood controllability.

A fourth object of the present invention is to provide a lightmodulation apparatus capable of stably controlling the transmittance, animage pickup apparatus using the light modulation apparatus, and methodsof driving the light modulation apparatus and the image pickupapparatus.

To achieve the fourth object, according to a fourth invention, there areprovided a light modulation apparatus including a liquid crystal device,a detection unit for detecting the intensity of transmission lighthaving passed through the liquid crystal device or an environmentaltemperature of the liquid crystal device, a control circuit unit forsetting a target intensity of the transmission light depending on theenvironmental temperature of the liquid crystal device on the basis of adetection value supplied from the detection unit, and a drive signalgeneration unit for generating a drive signal used for generating thetarget intensity of the transmission light by the control circuit unit,and an image pickup apparatus including the light modulation apparatusdisposed on an optical path of an optical system of the image pickupapparatus.

The light modulation apparatus may further include a control circuitunit, and may be configured such that the transmittance may becontrolled by monitoring the transmission light, feeding back thedetection information to the control circuit unit, and adjusting theintensity of the transmission light at a constant value, or monitoringan environmental temperature of the liquid crystal device, feeding backthe detection information to the control circuit unit, comparing thedetection information with a predetermined characteristic value, andadjusting the intensity of the transmission light at a constant value.

The control circuit unit may generate each drive pulse having an ACwaveform, whose pulse height is modulated, or each drive pulse whosepulse width or pulse density is modulated.

The light modulation apparatus may be configured such that the pulsewidth of each drive pulse having a basic waveform is modulated and thepulse height of the drive pulse is controlled depending on theenvironmental temperature of the liquid crystal device, or the pulseheight of each drive pulse having a basic waveform is modulated and thepulse width of the drive pulse is modulated depending on theenvironmental temperature of the liquid crystal device.

The light modulation apparatus may further include a drive circuit unit,and may be configured such that each drive pulse may be generated insynchronization of a clock generated by the drive circuit unit.

With the above configurations of the fourth invention, an intensity oftransmission light of the liquid crystal device for light modulation oran environmental temperature of the liquid crystal device is detected, atarget intensity of transmission light depending on the environmentaltemperature of the liquid crystal device is set on the basis of thedetected intensity of the transmission light or environmentaltemperature, and a specific drive signal for realizing the targetintensity of transmission light is generated, and accordingly, it ispossible to realize the drive of the liquid crystal device whileeliminating the effect of the environmental temperature as much aspossible, and to drive the light modulation apparatus in such a mannerthat a target transmittance can be usually obtained by performing thetemperature correction independently from the control of thetransmittance.

To achieve the fourth object, according to the present invention, thereare also provided methods of controlling a light modulation apparatusand an image pickup apparatus, each including the step of driving aliquid crystal device by detecting the intensity of transmission lighthaving passed through the liquid crystal device or an environmentaltemperature of the liquid crystal device, setting a target intensity ofthe transmission light depending on the environmental temperature of theliquid crystal device on the basis of a detection value supplied fromthe detection unit, and generating a drive signal used for generatingthe target intensity of the transmission light.

The methods of driving the light modulation apparatus and image pickupapparatus according to the fourth invention are each advantageous indriving the light modulation apparatus and image pickup apparatus with agood controllability.

The above-described first, second, third, and fourth inventions may befurther configured as follows:

Each drive electrode of the liquid crystal device may be formed over theentire region of at least an effective light transmission portion. Withthis configuration, the transmittance over the entire width of aneffective optical path can be collectively, accurately controlled bycontrol of the pulse width of each drive pulse to be applied between thedrive electrodes thus formed.

In the guest-host type liquid crystal device used for the lightmodulation apparatus, the host material may be a negative or positivetype liquid crystal having a negative or positive type dielectricconstant anisotropy, and the guest material may be a positive ornegative type dichroic dye molecular material having a positive ornegative type light absorption anisotropy.

The polarizing plate may be disposed in a movable portion of amechanical iris, and may be moved in and from the optical path byoperating the movable portion of the mechanical iris.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic views showing an operational principle ofa related art light modulation apparatus using a positive type liquidcrystal and FIG. 1C is a graph showing a rectangular waveform of avoltage applied to the liquid crystal;

FIGS. 2A and 2B are graphs showing a relationship between atransmittance of the apparatus shown in FIGS. 1A to 1B and a voltageapplied thereto, wherein FIG. 2A shows the relationship in a voltagerange of 0 to 10 V and FIG. 2B shows the relationship in a voltage rangeof 0 to 20 V;

FIG. 3A and 3B are schematic views showing an operational principle of alight modulation apparatus using a negative type guest-host liquidcrystal according to the present invention and FIG. 3C is a graphshowing a rectangular waveform of a voltage applied to the liquidcrystal;

FIGS. 4A and 4B are graphs showing a relationship between atransmittance of the apparatus shown in FIGS. 3A to 3B and a voltageapplied thereto, wherein FIG. 4A shows the relationship in a voltagerange of 0 to 10 V and FIG. 4B shows the relationship in a voltage rangeof 0 to 20 V;

FIGS. 5A to 5C are schematic views showing a parallel rubbing process,an anti-parallel rubbing process, and a one-side rubbing process forrubbing a liquid crystal device, respectively;

FIGS. 6A to 6C are graphs each showing a relationship between atransmittance of the light modulation apparatus including the liquidcrystal device shown in FIGS. 3A to 3C, which device is rubbed by eachof the parallel rubbing process, anti-parallel rubbing process, andone-side rubbing process, and a voltage applied to the liquid crystaldevice, respectively;

FIG. 7 is a graph showing a gap dependence on an initial transmittanceof the light modulation apparatus including the liquid crystal deviceshown in FIGS. 3A and 3B, which device is rubbed by the anti-parallelrubbing process;

FIGS. 8A to 8D are graphs each showing a gap dependence on a responsespeed of the light modulation apparatuses including the liquid crystaldevice shown in FIGS. 3A and 3B, which device is rubbed by each of theanti-parallel rubbing process and one-side rubbing process;

FIG. 9 is a diagram showing four relationships each between atransmittance flicker and a waveform of a drive pulse including each offour kinds of pulse resting periods, which drive pulse is applied to thelight modulation apparatus shown in FIGS. 3A and 3B;

FIG. 10 is a graph showing a relationship between a transmittance of thelight modulation apparatus shown in FIGS. 3A and 3B and a pulse width ofa drive pulse applied thereto, which drive pulse has each of pulseheights of 5 V and 10 V;

FIG. 11 is a diagram showing three kinds of modulated waveforms of drivepulses applied to the light modulation apparatus shown in FIGS. 3A and3B;

FIG. 12 is a graph illustrating a relaxation stage of a negative typeliquid crystal of the light modulation apparatus shown in FIGS. 3A and3C;

FIGS. 13A to 13C are graphs each showing a response characteristic ofthe light modulation apparatus using the negative type liquid crystalshown in FIGS. 3A and 3B;

FIG. 14 is a graph showing a relationship between a transmittance of thelight modulation apparatus shown in FIGS. 3A and 3C and each drive pulsemodulated in each of pulse width and pulse height;

FIG. 15 is a graph showing a relationship between a duty ratio and atransmittance of the light modulation apparatus shown in FIGS. 3A and 3Cto which each drive pulse modulated in each of pulse width and pulsedensity is applied;

FIGS. 16A to 16D are diagrams showing waveforms of four kinds of drivepulses, whose pulse widths are differently modulated, to be applied tothe light modulation apparatus shown in FIGS. 3A and 3C, and FIG. 16E isa graph showing a relationship between an intensity of the transmittanceand a pulse width of each drive pulse;

FIG. 17 is a graph showing an intensity of the transmittance of thelight modulation apparatus shown in FIGS. 3A to 3C and the number ofpositive and negative drive pulses applied thereto;

FIGS. 18A to 18D are schematic views each showing an alignment stage ofdirectors of light crystal molecules of the light modulation apparatusshown in FIGS. 3A to 3C;

FIG. 19 is a diagram illustrating a defect in alignment of liquidcrystal molecules depending on a relationship between a transmittance ofthe light modulation apparatus shown in FIGS. 3A to 3C and a voltageapplied thereto;

FIG. 20 is a graph showing comparative data on a change in transmittanceof the light modulation apparatus shown in FIGS. 3A to 3C depending on avoltage applied thereto;

FIGS. 21A and 21B are graphs each showing a relationship between atransmittance of the light modulation apparatus shown in FIGS. 3A to 3Cand each drive pulse whose pulse height is modulated in two-steps,wherein a waveform of the drive pulse is shown on the lower side of thefigure;

FIG. 22 is a graph showing a relationship between a transmittance of thelight modulation apparatus shown in FIGS. 3A to 3C and each drive pulsewhose pulse height is modulated in two-steps, wherein a waveform of thedrive pulse is shown on the lower side of the figure;

FIG. 23 is a diagram showing a waveform of each of various kinds ofdrive pulses applied to the light modulation apparatus shown in FIGS. 3Ato 3C;

FIG. 24 is a graph showing comparative data on a change in atransmittance of the light modulation apparatus shown in FIGS. 3A to 3Cand each drive pulse whose pulse height is modulated in a single step;

FIG. 25 is a graph showing a relationship between a transmittance of thelight modulation apparatus shown in FIGS. 3A to 3C and each drive pulsewhose pulse width is modulated in two-steps, wherein a waveform of thedrive pulse is shown on the lower side of the figure;

FIG. 26 is a graph showing a relationship between a transmittance ateach of different temperatures of a light modulation apparatus shown inFIGS. 3A to 3C and a pulse height of each drive pulse applied thereto;

FIG. 27 is a graph showing a relationship between pulse heights of drivepulses at different temperatures applied to the light modulationapparatus shown in FIGS. 3A to 3C;

FIG. 28 is a graph showing a compensation difference between voltages at65° C. and 23.5° C. applied to the light modulation apparatus shown inFIGS. 3A to 3C for obtaining the same transmittance;

FIG. 29 is a graph showing a compensation difference between voltages at65° C. and 25° C. applied to the light modulation apparatus shown inFIGS. 3A to 3C for obtaining the same transmittance;

FIG. 30 is a graph showing a relationship between a transmittance ateach of different temperatures of the light modulation apparatus shownin FIGS. 3A to 3C and a pulse height of each drive pulse appliedthereto;

FIG. 31 is a graph showing a relationship between a transmittance ateach of two temperatures of the light modulation apparatus shown inFIGS. 3A to 3C and a pulse height of each drive pulse applied thereto;

FIG. 32 is a graph showing a relationship between pulse heights of drivepulses at different temperatures applied to the light modulationapparatus shown in FIGS. 3A to 3C;

FIG. 33 is a schematic side view showing a configuration of a lightmodulation apparatus of the present invention;

FIG. 34 is a front view of a mechanical iris provided in the lightmodulation apparatus shown in FIG. 33;

FIGS. 35A to 35C are schematic partial enlarged views illustrating anoperation of the mechanical iris near an effective optical path of thelight modulation apparatus shown in FIG. 33;

FIG. 36 is a schematic sectional view of a camera system in which thelight modulation apparatus shown in FIG. 33 is assembled;

FIG. 37 is a diagram showing an algorithm for controlling atransmittance of the camera system shown in FIG. 36; and

FIG. 38 is a block diagram of the camera system including a drivecircuit shown in FIG. 36.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of a light modulation apparatus ofthe present invention will be described with reference to theaccompanying drawings.

First Embodiment

Referring to FIGS. 3A to 3C, there is shown a light modulation apparatusaccording to a first embodiment of the present invention, whichapparatus includes a guest-host type liquid crystal cell (GH cell) 12containing a host material 13 and a guest material 4, and a polarizingplate 11 disposed on the incident side of the GH cell 12.

A negative type liquid crystal having a negative dielectric constantanisotropy (Δ∈), produced by Merck under a trade name of MLC-6608, wasused as the host material 13. A positive type dichroic dye having apositive light absorption anisotropy (ΔA), produced by BDH under a tradename of D5, was used as the guest material 4.

With respect to the light modulation apparatus configured as describedabove, a change in transmittance (expressed in percentage based on thetotal quantity of light perfectly passing through the liquid crystalcell and the polarizing plate) of the light modulation apparatus wasmeasured in air by applying an operational voltage having a rectangularwaveform shown in FIG. 3C to the GH cell 12.

It should be noted that in this measurement, since the negative typeliquid crystal is used as the host material 13, light passes through theGH cell 12 when no voltage is applied thereto, and light is absorbed inthe GH cell 12 when a voltage is applied thereto.

As is apparent from the measured result shown in FIGS. 4A and 4B, anaverage transmittance (in air) of visual light is steeply changed orreduced from the maximum transmittance (about 75%) to several % with anincrease in applied voltage.

The reason the transmittance of the light modulation apparatus of thisembodiment is largely reduced with an increase in applied voltage is asfollows: when using the negative type host material, since theinteraction of liquid crystal molecules at the boundary between a liquidcrystal alignment film of the liquid crystal cell and the liquid crystalmolecules is very weak upon application of no voltage, light easilypasses through the liquid crystal cell when no voltage is appliedthereto, and directors (alignment vectors) of the liquid crystalmolecules become easy to change when a voltage is applied thereto.

For comparison, a change in transmittance of a light modulationapparatus shown in FIGS. 1A to 1C was measured in the same manner asthat described above. As shown in FIGS. 1A to 1C, the light modulationapparatus includes a guest-host type liquid crystal cell (GH cell) 2containing a host material 3 and a guest material 4, and a polarizingplate 1 disposed on the incident side of the GH cell 2.

A positive type generalized liquid crystal having a positive dielectricconstant anisotropy (Δ∈), produced by Merck under a trade name ofMLC-6849, was used as the host material 3, and the same positive typedichroic dye D5 (trade name, produced by BDH) as that used in the firstembodiment was used as the guest material 4.

With respect to the light modulation apparatus configured as describedabove, a change in transmittance of the light modulation apparatus wasmeasured by applying an operational voltage having a rectangularwaveform shown in FIG. 1C to the GH cell 2.

It should be noted that in this measurement, since the positive typeliquid crystal is used as the host material 3, light is absorbed in theGH cell 2 when no voltage is applied thereto, and light passes throughthe GH cell 2 when a voltage is applied thereto.

As is apparent from the measured result shown in FIGS. 2A and 2B, anaverage transmittance of visual light is slowly changed or increasedwith an increase in applied voltage, and reaches the maximumtransmittance (about 60%) when a voltage of 20 V is applied to theliquid crystal cell 2.

The reason the transmittance of the light modulation apparatus of thiscomparative example is slowly changed with an increase in appliedvoltage and the maximum transmittance thereof is relatively small is asfollows: when using the positive type host material, since theinteraction of liquid crystal molecules at the boundary between a liquidcrystal alignment film of the liquid crystal cell and the liquid crystalmolecules is strong upon application of no voltage, there may remainliquid crystal molecules, whose directors do not change or are not easyto change, even when a voltage is applied thereto.

As described above, the light modulation apparatus including GH cell 12using the negative type host material according to the first embodimentis advantageous in that since the maximum transmittance can be increasedup to about 75%, the apparatus can be designed to be operable in a hightransmittance region, and since the transmittance can be steeplychanged, the apparatus can easily control the transmittance by anoperational voltage.

The combination of the host material and guest material constituting theGH cell 12 can be variously changed, the examples of which may include acombination of a negative host material (Δ∈<0) and a positive type guestmaterial (ΔA>0); a combination of a negative host material (Δ∈<0) and anegative type guest material (ΔA<0); a combination of a positive typehost material (Δ∈>0) and a positive type guest material (ΔA>0); and acombination of a positive type host material (Δ∈>0) and a negative typeguest material (ΔA<0).

Although in the GH cell 12, a drive electrode, typically an ITO (IndiumTin Oxide: Indium Oxide doped with Tin) electrode is provided in solidon a substrate surface, it may be divided to be used in a segment modeor a matrix mode.

Examples of negative host materials (Δ∈<0) usable for the lightmodulation apparatus according to the present invention may includecompounds having the following molecular structures:

EXAMPLE 1

Molecular structure Δε C N I

−4.0 +45 +101 +

−4.2 +56 +113 +

−22 +85.8 SA (+52.0) +

−18 +133.5 +143.5 +

−8 +24   66 +

EXAMPLE 2

<Other Basic Skeletons>

R, R¹, R², and L express normal chain or branched alkyl group, alkoxygroup, alkenyl group, fluoroalkoxy group, fluoroalkenyl group, —CNgroup, etc.

EXAMPLE 3

EXAMPLE 4

EXAMPLE 5

EXAMPLE 6

EXAMPLE 7

Examples of negative host materials usable for the light modulationapparatus according to the present invention may include the followingcommercially available compounds:

EXAMPLE 1

MLC-6608 (produced by Merck) S-N shift <−30.0 degree Cleaningtemperature +90.0° C. Rotational viscosity ν₁ 20° C. 186.0 mPa · sOptical anisotropy Δn 0.0830 +20° C. 589.3 nm n_(e) 1.5586 n_(o) 1.4756Dielectric anisotropy Δε −4.2 +20° C. 1.0 kHz ε⊥ 7.8 ε// 3.6 Elasticconstant K₁₁ 16.7 pN +20° C. K₃₃ 18.1 pN K₃₃/K₁₁ 1.08 Stability at lowtemperature −30° C. 1000 h cr

EXAMPLE 2

MLC-2039 (produced by Merck) Cleaning temperature +91.0° C. Rotationalviscosity ν₁ 20° C. 163.0 mPa · s Optical anisotropy Δn 0.0821 +20° C.,589.3 nm n_(e) 1.5575 n_(o) 1.4754 Dielectric anisotropy Δε −4.1 +20°C., 1.0 kHz ε⊥ 7.6 ε// 3.5

EXAMPLE 3

MLC-2038 (produced by Merck) Cleaning temperature +80.0° C. Flowviscosity ν 20° C. 29 mm²s⁻¹ 0° C. 128 mm²s⁻¹ −20° C. 1152 mm²s⁻¹ −30°C. 6369 mm²s⁻¹ Rotational viscosity ν₁ 20° C. 179.0 mPa · s Opticalanisotropy Δn 0.1032 +20° C. 589.3 nm n_(e) 1.5848 n_(o) 1.4816Dielectric anisotropy Δε −5.0 +20° C. 1.0 kHz ε⊥ 9.0 ε// 4.0 Elasticconstant K₁₁ 13.8 pN +20° C. K₃₃ 18.1 pN K₃₃/K₁₁ 1.31 Stability at lowtemperature −30° C. 48 h cr −20° C. 432 h cr

EXAMPLE 4

MLC-2037 (produced by Merck) S-N shift <−20.0° C. Cleaning temperature+71.0° C. Rotational viscosity ν₁ 20° C. 132.0 mPa · s Opticalanisotropy Δn 0.0649 +20° C. 589.3 nm n_(e) 1.5371 n_(o) 1.4722Dielectric anisotropy Δε −3.1 +20° C. 1.0 kHz ε⊥ 6.7 ε// 3.6 Stabilityat low temperature −20° C. 1000 h cr

The above compounds may be used singularly or in combination to exhibitthe neumatic property in a real service temperatural range.

Examples of dichroic dye molecular materials usable for the lightmodulation apparatus of the present invention may include compoundshaving the following molecular structures:

EXAMPLE 1

degree of λm co- di- molecular structure (nm) lor chroism D5

590 B 5.3 D35

553 P 6.5 L-dye B

641 B 9.2 G209

687 B 9.5 G168

574 B 10.6 G165

595 B 10.3 G224

574 V 9.7 G205

507 R 11.4 G232

450 Y 12.1 B: blue, P: purple, V: violet, R: red, Y: yellow D5, D35:produced by BDH L-dye B: produced by Roche Others: produced by JapanPhotosensitive Pigment Laboratory

EXAMPLE 2

^(λ)max (nm) (in liquid S pigment structure crystal) (value at λmax)

450 0.79

440 0.78

542 0.75

548 0.78

573 0.77

610 0.83

464 0.80

520 0.77

0.76

EXAMPLE 3

^(λ)max (nm) S pigment structure (in liquid crystal) (value at λmax)

638 0.78

638 0.77

627 0.76

640 0.77

668 0.74

565 −0.377

548 −0.33Second Embodiment

In this embodiment, a response speed of a light modulation apparatusupon application of a voltage thereto was examined.

The response speed of a light modulation apparatus upon application of avoltage thereto varies depending on not only the kind of drive of theapparatus but also means used for producing a liquid crystal device, forexample, a rubbing process. The rubbing process involves forming a filmmade from a high polymer such as polyimide or polyvinyl alcohol on asubstrate, and rubbing the film with cloth, thereby uniformly aligningliquid crystal molecules in the rubbing direction [D. W. Berrenan, Mol.Cryst. & Liq. Cryst., 23.215(1973)].

Examples of the rubbing processes include a parallel rubbing process, ananti-parallel rubbing process, and a one-side rubbing process. Theparallel rubbing process shown in FIG. 5A involves rubbing bothalignment films formed on upper and lower substrates in such a mannerthat the rubbing direction on the upper alignment film is parallel tothat on the lower alignment film. The anti-parallel rubbing processshown in FIG. 5B involves rubbing both alignment films formed on upperand lower substrates in such a manner that the rubbing direction on theupper alignment film is anti-parallel to that on the lower alignmentfilm. The one-side rubbing process shown in FIG. 5C involves rubbingonly an alignment film formed on one of upper and lower substrates.

Since alignment of liquid crystal molecules largely differs depending ona material of an alignment film and a film formation condition of thealignment film, it is required to select an alignment film materialsuitable for a liquid crystal material and to examine a film formationcondition and a rubbing condition suitable for the liquid crystalmaterial. In this regard, according to the present invention, it ispossible to determine conditions, particularly, a rubbing processcondition suitable for a liquid crystal composition used for a lightmodulation apparatus.

At first, a change in transmittance depending on a voltage applied to alight modulation apparatus including a liquid crystal device rubbed bythe parallel rubbing process was examined as follows:

A light modulation apparatus having the same basic configuration as thatshown in FIGS. 3A to 3C, that is, including a GH cell 12 containing ahost material 13 and a guest material 4 and a polarizing plate 11disposed on the incident side of the GH cell 12, was prepared. In thisexample, a negative type generalized liquid crystal having a negativedielectric constant anisotropy (Δ∈), produced by Merck Incorporationunder a trade name of MLC-2039, was used as the host material 13; thesame positive type dichroic dye as that used in the first embodiment,produced by BDH Incorporation under the trade name of D5, was used asthe guest material 4; and the GH 12 was rubbed by the parallel rubbingprocess shown in FIG. 5A. With respect to each of three samples of thelight modulation apparatuses thus prepared, a change in transmittance ofthe sample was measured by applying an operational voltage having arectangular waveform shown in FIG. 3C to the GH cell 12.

From the result shown in FIG. 6A, it becomes apparent that thetransmittance is not dependent on the operational voltage, and is notreduced to a specific value. This means that the parallel rubbingprocess is unsuitable for alignment of the negative type liquid crystalmolecules.

At second, a change in transmittance depending on a voltage applied to alight modulation apparatus including a liquid crystal device rubbed bythe anti-parallel rubbing process was examined as follows:

A light modulation apparatus having the same basic configuration as thatshown in FIGS. 3A to 3C, that is, including a GH cell 12 containing ahost material 13 and a guest material 4 and a polarizing plate 11disposed on the incident side of the GH cell 12, was prepared. In thisexample, the same negative type liquid crystal as that used in the firstembodiment, produced by Merck Incorporation under the trade name ofMLC-6608, was used as the host material 13; the same positive typedichroic dye as that used in the first embodiment, produced by BDHIncorporation under the trade name of D5, was used as the guest material4; and the GH 12 was rubbed by the anti-parallel rubbing process shownin FIG. 5B. With respect to each of three samples of the lightmodulation apparatuses thus prepared, a change in transmittance of thesample was measured by applying an operational voltage having arectangular waveform shown in FIG. 3C to the GH cell 12.

From the result shown in FIG. 6B, it becomes apparent that an averagetransmittance (in air) of visual light is steeply changed or reducedfrom a maximum transmittance (75%) to several % with an increase inoperational voltage.

In this way, the transmittance of the GH cell rubbed by theanti-parallel rubbing process exhibits a high voltage dependence, thatis, it can be controlled on the basis of a voltage applied to the GHcell, and further, the range of the transmittance controllable by avoltage is enlarged. In addition, the reason why the transmittance issteeply reduced with an increase in operational voltage and the maximumtransmittance is high in FIG. 6B may be considered as follows: namely,in the case of using the negative type host material, since theinteraction of liquid crystal molecules at the boundary between a liquidcrystal alignment film of the liquid crystal cell and the liquid crystalmolecules is very weak upon application of no voltage, light is easy topass through the liquid crystal cell when no voltage is applied thereto,and directors of the liquid crystal molecules become easy to change whena voltage is applied thereto.

At third, a change in transmittance depending on a voltage applied to alight modulation apparatus including a liquid crystal device rubbed bythe one-side rubbing process was examined as follows:

A light modulation apparatus having the same basic configuration as thatshown in FIGS. 3A to 3C, that is, including a GH cell 12 containing ahost material 13 and a guest material 4 and a polarizing plate 11disposed on the incident side of the GH cell 12, was prepared. In thisexample, the same negative type liquid crystal as that used in the firstembodiment, produced by Merck Incorporation under the trade name ofMLC-6608, was used as the host material 13; the same positive typedichroic dye as that used in the first embodiment, produced by BDHIncorporation under the trade name of D5, was used as the guest material4; and the GH 12 was rubbed by the one-side rubbing process shown inFIG. 5C. With respect to each of three samples of the light modulationapparatuses thus prepared, a change in transmittance of the sample wasmeasured by applying an operational voltage having a rectangularwaveform shown in FIG. 3C to the GH cell 12.

From the result shown in FIG. 6C, it becomes apparent that an averagetransmittance (in air) of visual light is steeply changed or reducedfrom a maximum transmittance (about 75%) to several % with an increasein operational voltage.

In this way, the transmittance of the GH cell rubbed by theanti-parallel rubbing process exhibits a high voltage dependence, thatis, it can be controlled on the basis of a voltage applied to the GHcell. In addition, the reason why the transmittance is steeply reducedwith an increase in operational voltage and the maximum transmittance ishigh in FIG. 6C may be considered as follows: namely, in the case ofusing the negative type host material, since the interaction of liquidcrystal molecules at the boundary between a liquid crystal alignmentfilm of the liquid crystal cell and the liquid crystal molecules is veryweak upon application of no voltage, light is easy to pass through theliquid crystal cell when no voltage is applied thereto, and directors ofthe liquid crystal molecules become easy to change when a voltage isapplied thereto.

Next, a factor typically a pre-tilt angle for determining an initialtransmittance (upon application of no voltage) of the GH cell rubbed bythe one-side rubbing process was examined.

FIG. 7 is a graph showing a pre-tilt angle dependence on an initialtransmittance (upon the off state of voltage) of the light modulationapparatus shown in FIGS. 3A to 3C. The pre-tilt angle is defined as anangle at which liquid crystal molecules are tilted along the tiltdirection of main chains of a film made from a high polymer such aspolyimide or polyvinyl alcohol at the rubbing step. The pre-tilt angle,therefore, has a strong relation with the rubbing process.

From the result shown in FIG. 7, it becomes apparent that when a designcell gap is 6 μm or more, the initial transmittance is not dependent onthe pre-tilt angle. In other words, if the cell gap of the liquidcrystal device of the light modulation apparatus is in a range of 5 μmor less, the transmittance can be adjusted by the alignment process.

In the case of using the GH cell rubbed by the anti-parallel process., aresult similar to that shown in FIG. 7 was obtained.

An effect of the anti-parallel rubbing process exerted on a responsespeed was compared with that of the one-side rubbing process exerted onthe response speed, as follows: namely, a relationship between aresponse speed of the light modulation apparatus including the liquidcrystal device rubbed by each of the anti-parallel rubbing process andthe one-side rubbing process controllable by a voltage and a cell gapwas examined by applying a voltage to the liquid crystal device in eachof a large-scale drive mode (drive wave form: 0–5 V at 1 kHz) and anintermediate-scale drive mode (drive waveform: 2–3 V at 1 kHz) at eachof 22° C. and 65° C. The results are shown in FIGS. 8A to 8D. In thesefigures, for easy comparison, the response speed is expressed in anabsorbance changed per unit response time.

From the results shown in FIGS. 8A and 8B, it becomes apparent that thegap dependence on the response speed in the intermediate-scale drivemode (2–3 V) does not appear in the low temperature environment, 22° C.(see FIG. 8A) while the gap dependence on the response speed in thelarge-scale drive mode (0–5 V) appears at 22° C. (see FIG. 8B); and theresponse speed of the apparatus including the cell rubbed by theanti-parallel rubbing process is higher than that of the apparatusincluding the cell rubbed by the one-side rubbing process at 22° C.,irrespective of the presence or absence of the gap dependence.

From the results shown in FIGS. 8C and 8D, it becomes apparent that thegap dependence on the response speed appears in each of theintermediate-scale drive mode (2–3 V) and the large-scale drive mode(0–5 V) in the high temperature environment, 65° C.; and the responsespeed of the apparatus including the cell rubbed by the anti-parallelrubbing process is higher than that of the apparatus including the cellrubbed by the one-side rubbing process in the intermediate-scale drivemode at 65° C. while the response speed of the apparatus in thelarge-scale drive mode (0–5 V) at 65° C. is not dependent on the kind ofrubbing process.

In this way, the response speed of the light modulation apparatusincluding the liquid crystal device rubbed at a pre-tilt angle, exertingan effect on the initial transmittance, in the range of 5 μm or less bythe anti-parallel rubbing process can be made higher than that of theapparatus including the liquid crystal device rubbed at the samepre-tilt angle by the one-side rubbing process. The reason for this maybe considered that in the liquid crystal device rubbed by theanti-parallel rubbing process, directors of aligned liquid crystalmolecules are easy to change by an electric field applied thereto.

Third Embodiment

In this embodiment, the control of a transmittance of a light modulationapparatus by modulating a pulse width or a pulse density of drive pulsesapplied to a GH cell of the apparatus was examined.

In particular, the modulation of a pulse width of each drive pulse forcontrolling a transmittance of the light modulation apparatus iseffective to independently perform the control of the transmittance andthe compensation of the transmittance. Specifically, the transmittanceis normally controlled by modulating the pulse width of each drive pulseon the basis of a normal feedback control signal and the transmittanceis compensated by modulating the pulse height of the drive pulse on thebasis of a temperature correction feedback signal, or the transmittanceis normally controlled by modulating the pulse height of each drivepulse on the basis of a normal feedback control signal and thetransmittance is compensated by modulating the pulse width of the drivepulse on the basis of a temperature correction feedback signal.

(1) Basic Rectangular Waveform of Drive Pulse and Flicker of LightModulation Apparatus

The waveform of a voltage to be applied to the GH 12 is a rectangularwaveform as shown in FIG. 3C; however, it may be a trapezoidal waveformor sine waveform. When each drive pulse shown in FIG. 3C is applied to aliquid crystal device, directors of liquid crystal molecules are changeddepending on a differential potential between both electrodes, tothereby control the transmittance of light. Accordingly, thetransmittance is generally controlled on the basis of a pulse height (orpulse voltage) of the drive pulse.

The control of such a pulse height of each drive pulse, however, must bebasically subjected to D/A conversion, and further, it is difficult tohighly accurately control the pulse height, with a result that thecontrol of the pulse height causes a problem in increasing the circuitcost.

By the way, an electro-optical response of a nematic liquid crystalmaterial is as slow as several ms at minimum and several hundreds ms atmaximum. From this viewpoint, the present inventor has examined asuitable basic pulse generation period of drive pulses for stablycontrolling the transmittance of a material having such a responsecharacteristic by adopting the mode of modulating the pulse width ofeach drive pulse applied thereto.

A test for determining the basic pulse generation period was performedby applying drive pulses to a liquid crystal device of a lightmodulation apparatus in the order of 0 V→5 V→0 V→−5 V→0 V . . . as shownin FIG. 9, and a variation in transmittance of the apparatus wasobserved by changing the width of each pulse, particularly, the width ofeach resting pulse (0 V).

As is apparent from the results shown in FIG. 9, a flicker oftransmittance appears, that is, the transmittance is unstable when theresting pulse period is 300 μs or more, and any flicker of transmittancedoes not appear when the resting pulse period is 200 μs or less.

Accordingly, the pulse width of each pulse applied to the liquid crystaldevice of the light modulation apparatus should be modulated in such amanner that the resting pulse period does not exceed about 200 μs. Sincethe response speed of a liquid crystal is dependent on the kind of theliquid crystal and an environmental temperature, the resting pulseperiod must be set at such a value as not to cause a flicker oftransmittance under service conditions. Further, to obtain stableoptical characteristics of a liquid crystal device, it is effective tocontrol the pulse width of each drive pulse on the basis of anenvironmental temperature feedback signal.

(2) Modulation of Pulse Width

As the result of the above-described examination, the basic pulsegeneration period was set at 100 μs, and the pulse width (PW) wasmodulated within this basic pulse generation period. FIG. 10 shows achange in transmittance of the light modulation apparatus depending on apulse width PW of each drive pulse applied to the liquid crystal deviceof the apparatus with a pulse height of the drive pulse set at aconstant value, for example, each of 5 V and 10 V. In addition, thetransmittance is expressed in percentage based on the total quantity oflight passing through the liquid crystal cell and the polarizing plateupon application of no voltage applied to the liquid crystal device.

As is apparent from FIG. 10, the transmittance can be easily controlledby modulating the pulse width of each drive pulse within the pulsegeneration period of 100 μs under the condition that the pulse height ofthe drive pulse is set at each of 5 V and 10V. This is because directorsof liquid crystal molecules are changed by an electric field energycorresponding to the pulse width of each drive pulse, and thereby thealignment of the liquid crystal molecules is controlled. From the resultshown in FIG. 10, it is also found that the transmittance can be freelycontrolled by the combination of the pulse height and the pulse width ofeach drive pulse. This means that the limitation of gradation due to thelimitation of minimum clock can be eliminated, that is, the resolutionof the gradation control can be increased by controlling a pulse heightin digital as a lower bit and simultaneously modulating the pulse widthas an upper bit, or by controlling a pulse width in digital as a lowerbit and simultaneously modulating the pulse height as an upper bit. Themodulation of the pulse width of each drive pulse has a further merit interms of cost because the pulse width of the basic waveform of the drivepulse can be modulated in synchronization of a clock generated by aperipheral circuit of an apparatus including the light modulationdevice.

FIGS. 11A and 11B show two waveforms of each drive pulse modulated inpulse width. In the waveform shown in FIG. 11A, the pulse is applied atthe start of the basic pulse generation period, and in the waveformshown in FIG. 11B, the pulse is applied after an elapse of a specificdelay time since the start of the basic pulse generation period. Theeffect of the drive pulse having the waveform in FIG. 11A is the same asthat of the drive pulse having the waveform in FIG. 11B. With respect tothe waveform in FIG. 11B, the pulse may be applied after an elapse ofthe delay time since each basic pulse generation period. Further, thewaveform in FIG. 11B may be combined with the waveform in FIG. 11A. Inaddition, a necessary number of drive pulses may be applied within thebasic pulse generation period. FIG. 11C also shows the modulation of thepulse density, in which the numerical density of pulses within the basicpulse generation period is modulated.

FIG. 12 shows a relaxation stage of a negative type liquid crystal usedas a host material, for example, of the GH cell shown in FIGS. 3A to 3C.

The relaxation stage of a negative type liquid crystal system isexpressed by the following equations:R=R ₁[1−exp (−T/τ₁)]+R ₂[1−exp−(T/τ₂)²]Basic Period≦−(Relaxation Time)×In (0.98)T=−τ×In [1−2/100]Relaxation Stage at Relaxation Time τ₁ →T=300 to 400 μs

The values R₁, τ₁, R₂, and τ₂ are shown in FIG. 12, for example, R₁=78%,τ₁=15.8 ms, R₂=22%, and τ₂=17.6 ms at the relaxation state of 3 V→0 V.

From the result shown in FIG. 12, it becomes apparent that therelaxation time after a pulse voltage is applied and then turn off ischanged depending on the pulse height, and more specifically, therelaxation time becomes longer as the pulse height becomes higher.

FIG. 13A shows the intensity of the transmission light of the lightmodulation apparatus in a relaxation stage similar to that shown in FIG.12, and FIGS. 13B and 13C each show a variation in intensity of thetransmission light of the light modulation apparatus in the relaxationstage. From the data on the variation in intensity of the transmissionlight in a region of 0 to 5 ms in which relaxation is linearly generated(FIG. 13B), it becomes apparent that the off time of 300 μs or less isrequired to specify the variation in a range of 1% or less, and the offtime of 420 μs or less is required to specify a variation in a range of2% or less.

The above off time corresponds to the resting pulse period foreliminating a flicker of transmittance shown in FIG. 9, so that thebasic pulse generation period can be set at, for example, 100 μs underthe condition that a variation in intensity of the transmission light inthe range of 2% or less is allowable.

The variation in intensity of the transmission light in the range of 2%or less is set on the basis of the image pickup specification of theexisting CCD (which will be described later).

In the image pickup of the CCD, even if there occurs a variation inintensity of the transmission light in the range of more than 2%, it isestimated that flicker little appears upon usual operation of the CCDbecause the image pickup of the CCD is based on an average of lightquantity accumulated in a field period; however, the dynamic range ofthe transmittance control is degraded, and if a shutter is used, theopen time of the shutter is not proportional to the light quantity, tocause a problem in terms of control. As a result, in the image pickup ofthe CCD, it may be desirable to specify the variation in intensity oftransmission light in the range of 2% or less.

If the basic pulse generation period exceeds the field period of theCCD, a flicker may appear upon usual operation of the CCD. Accordingly,to carry out the modulation of the pulse width of each drive pulse, itis essential to set the basic pulse generation period within the fieldperiod of the CCD.

(3) Comparison Between Modulation of Pulse Width and Modulation of PulseHeight

FIG. 14 shows a graph for comparing the characteristic of a conventionalpulse height modulation (PHM) mode and the characteristic of a pulsewidth modulation (PWM) mode. In this graph, the abscissa indicates anaverage per unit time of absolute values of differential potentialsapplied between electrodes, which is taken as an equivalent voltage.

As is apparent from FIG. 14, as compared with the curve indicating thePHM mode, the curve indicating the PWM mode is lower in thresholdvoltage and is shifted on the lower voltage side as a whole. As aresult, according to the PWM mode, the transmittance can be controlledby a lower voltage, to reduce the power consumption, and since thetransmittance is relatively moderately changed depending on a voltage,it is easy to be controlled by the voltage, to improve the gradation.

In this way, the pulse width modulation (PWM) mode has the followingadvantages:

(1) to reduce a threshold voltage;

(2) to increase the number of gradation of the transmittance level, andhighly accurately control a transmittance; and

(3) reduce a circuit cost because of no D/A conversion.

(4) Modulation of Pulse Width and Modulation of Pulse Density

A pulse density modulation (PDM) mode used in place of modulation of apulse height of each drive pulse was compared with the above-describedPWM mode. In the PDM mode, the number of pulses generated per unit timeis modulated, and in general, the pulses, each having a very shortwidth, are frequently generated per unit time.

As shown in FIG. 15, the drive characteristic of the PWM mode is verysimilar to that of the PDM mode; however, the PWM mode is superior inpower consumption to the PDM mode because the PWM mode is smaller thanthe PDM mode in terms of the amount of a current charged in a liquidcrystal cell per unit time. The PWM mode is also superior to the PDMmode in terms of impedance matching.

(5) Effect of Pulse Number

In the case of controlling a transmittance of a light modulationapparatus in the pulse width modulation mode, it is possible toeliminate the deviation of polarization of ions or the like in the lightmodulation apparatus by driving the apparatus in such a manner that anaverage per unit time of differential potentials (DC components) appliedbetween electrodes of a liquid crystal device of the apparatus becomesnearly zero, and hence to highly accurately control the transmittance ofthe apparatus.

For example, when two positive pulses and two negative pulses are, asshown in FIG. 16B, alternately applied to a basic drive waveform of FIG.16A, if an average per unit time of the number of the positive pulses isequal to that of the number of the negative pulses, it is possible tousually obtain the same drive characteristic of the transmittance.

As shown in FIG. 16C, the relationship between a transmittance and apulse width is not changed irrespective of the number (m=1, 2, . . . )of positive pulses and the number (m=1, 2, . . . ) of negative pulsesinsofar as the number (m=1, 2, . . . ) of the positive pulses is equalto the number (m=1, 2, . . . ) of the negative pulses.

As shown in FIG. 16D, the relationship between a transmittance and apulse width is not changed irrespective of the generation order ofpulses insofar as the number of the positive pulses is equal to that ofthe negative pulses. Further, it can be easily estimated that therelationship between a transmittance and a pulse width is not changedeven if pulse widths are individually modulated, insofar as are averageper unit time of the pulse widths is specified.

On the contrary, if the number of positive pulses is different from thatof negative pulses, the relationship between a transmittance and a pulsewidth is changed. Now, it is assumed that the number of negative pulsesis as large as k times the number of the positive pulses. If k=1, thedrive pulses (positive and negative pulses) are symmetrically appliedwith respect to 0 V, and in this case, the relationship between atransmittance and a pulse width is not changed. On the other hand, ifthe value of k becomes larger than 1, the drive pulses (positive andnegative pulses) are asymmetrically applied with respect to 0 V, and inthis case, the relationship between a transmittance and a pulse width ischanged, and more specifically, as shown in FIG. 17, the transmittancebecomes larger than a specific transmittance, thereby degrading thecontrollability of the transmittance. That is to say, the transmittanceis varied not depending on the value “m” but depending on the value “k”.

If the polarities of asymmetric pulses are instantly reversed, thetransmittance is temporarily reduced and is returned to the originaltransmittance after several seconds. Such a transient variation in theorder of seconds, observed as a flicker with a long period, may beconsidered to occur due to a deviation of movable ions in a liquidcrystal cell, which deviation may be caused by an average per unit timeof bias voltages.

As described above, to stably control the transmittance, it may bedesirable to symmetrically apply the drive pulses (positive and negativepulses) with respect to 0 V, that is, to make the number of the positivepulses equal to that of the negative pulses.

Fourth Embodiment

In this embodiment, the control of a transmittance of a light modulationapparatus by modulating stepwise the pulse width of each drive pulseapplied to a liquid crystal device of the apparatus was examined.

Rubbing Effect and Defect in Alignment

A test for examining the rubbing effect and a defect in alignment ofliquid crystal molecules was performed by using a light modulationapparatus including a liquid crystal cell shown in FIGS. 3A to 3C. Thecell was produced by making glass substrates, each having a transparentelectrode on the upper surface of which a liquid crystal alignment layerwas provided, face to each other with a specific gap put therebetweenand filling the gap with a guest-host liquid crystal in a reducedpressure. Each drive pulse having an AC rectangular waveform shown inFIG. 3C was applied to the cell of the apparatus, and a defect inalignment of liquid crystal molecules was observed.

If the liquid crystal alignment layer (not shown) is not subjected torubbing treatment, as shown in FIG. 18A, when a voltage is applied tothe cell, light crystal molecules are tilted with respect to thesubstrate plane and simultaneously, liquid crystal molecules and pigmentmolecules in a plane parallel to the glass substrate plane weredisturbed, with a result that non-uniformity of the transmittance occursin the substrate plane. To solve such a problem, as a known technique,the tilting direction of the liquid crystal molecules is previouslyspecified by rubbing the liquid crystal alignment layer as shown in FIG.18B, to uniformly tilt the liquid crystal molecules, thereby improvingthe in-plane uniformity of the liquid crystal molecules.

However, if a large drive voltage is applied with a single step to theliquid crystal device rubbed as described above, there occurs atransient state in which liquid crystal molecules are aligned indifferent directions, and if such a transient state continues for a timebeing long enough to exert an effect on the transmittance, there appearsin-plane non-uniformity in transmittance. In general, the transientstate disappears after an elapse of a certain time required forre-alignment of liquid crystal molecules and pigment molecules; however,in the worst case, the transient state may partially remain even afteran elapse of a long time.

To solve such a problem, as shown in FIG. 18D, a preparation voltagebeing low enough not to cause a defect in alignment of liquid crystalmolecules is applied to the liquid crystal device to tilt the liquidcrystal molecules to some extent, and then a final voltage being highenough to achieve a desired transmittance is applied to the liquidcrystal device, to control the transmittance in an in-plane uniformstate.

Transmittance and Defect in Alignment in One-step Drive Mode (1)

As shown in FIG. 19, in the case of applying a voltage (based on 0 V)with a single step for achieving the transmittance of 15% or less byeach of the pulse width modulation (PWM) or pulse height modulation(PHM) mode, there occur defects in alignment of liquid crystal moleculesand pigment molecules as described with reference to FIG. 18C. It shouldbe noted that, in the case of applying a voltage with a single step forachieve the transmittance of 15% or more by each of the PWM or PHM mode,there do not occur defects in alignment of liquid crystal molecules andpigment molecules.

FIG. 20 shows a change in transmittance of a light modulation apparatusand a frequency of defects of alignment depending on a pulse voltageapplied to a liquid crystal device of the apparatus in each of the PWMmode and PHM mode. As is apparent from this graph, the change intransmittance and the frequency of defects of alignment are dependent onthe pulse voltage, and in particular, as the pulse voltage is increasedup to 5 V or more (that is, the transmittance becomes 15% or less), thedefect in alignment of liquid crystal molecules is liable to occur.

Change in Transmittance in Two-step Drive Mode (1)

A method of driving a liquid crystal device by modulating the pulseheight of each AC pulse with two steps according to the presentinvention will be described.

In an example shown in FIG. 21A, a two-step drive mode in which apreparation voltage of 4 V was first applied for 90 ms and then a finalvoltage of 10 V was applied is performed in place of a single step drivemode of 0 V→10 V. With this two-step drive mode, the unstable change intransmittance as shown in FIG. 20 does not appear and instead a stablechange in transmittance appears. In an example shown in FIG. 21B, atwo-step drive mode, in which a preparation voltage of 4 V was firstapplied for 15 ms and then a final voltage of 10 V was applied, isperformed. Even in this example, the change in transmittance isstabilized. As described in the above examples, the pulse time width ateach step can be freely selected. In addition, the two-step drive modeshown in FIG. 21B may be suitable for quick response.

FIG. 22 shows a drive waveform similar to that shown in FIG. 21B indetail. In the two-step drive mode shown in this graph, at the firststep, positive and negative pulses (pulse height: 4.5 V, pulse width:500 μs) are alternately repeated by 15 times, and at the second step,positive and negative pulses (pulse height: 10 V, pulse width: 500 μs)are alternately repeated by 75 times. As a result, it becomes apparentthat if the positive and negative pulses (pulse height: 4.5 V, pulsewidth: 500 μs) are previously applied to the liquid crystal device for15 ms or more, there does not occur a defect in alignment even if adrive pulse (pulse height: 5 V or more) is then applied to the liquidcrystal device.

In this way, as the drive of the light modulation apparatus by applyingtwo-step pulses in accordance with the present invention, it is possibleto obtain the change in transmittance having a desired profile, toimprove the control accuracy of the transmittance, and to enhance thein-plane uniformity of the transmittance of the apparatus.

In this two-step drive mode, the pulse width and pulse height of apreparation pulse can be freely specified, and each of patterns shown inFIGS. 23A to 23E can be selected as the combination of preparation andfinal pulses. The drive pulse may be modulated in multi-steps more thantwo-steps in accordance with the need of the light modulation apparatus,and further a method of modulating only the pulse width with the pulseheight kept constant, for example, pulse width modulation mode, can beapplied as will be described later.

Transmittance and Defect in Alignment in One-step Drive Mode (2)

In the case of achieving the transmittance of 15% or less by applying adrive pulse modulated in pulse width with a pulse height is keptconstant, as described with reference to FIG. 18C, the change intransmittance is changed depending on the pulse height of the drivepulse. As shown in FIG. 24A, if the pulse height is set at 5 V, theredoes not occur an unstable change in transmittance due to a defect inalignment, and as shown in FIG. 24B, if the pulse height is set at 10 V,an unstable change (abnormal response) in transmittance due to a defectin alignment is liable to occur.

Change in Transmittance in Two-step Drive Mode (2)

To prevent the occurrence of a defect in alignment of liquid crystalmolecules, in particular, in the case where the pulse height of eachdrive pulse applied to the liquid crystal device is high, according tothe present invention, a pulse width of the above drive pulse ismodulated with two-steps. For example, in the case of applying a drivepulse whose pulse height of 10 V (based on 0 V), as shown in FIG. 25,positive and negative preparation pulses (pulse height: 10 V, pulsewidth: 20 μs) are applied for 15 ms, and then positive and negativepulses (pulse height: 10 V, pulse width: 100 μs) are applied. In thiscase, as shown in FIG. 25, the change in transmittance becomes stable.The combination of pulses different in pulse width is not limited tothat described above. The pulse width of each drive pulse may bemodulated in two or more steps, and the pulse height of the drive pulsemay be variously changed.

Fifth Embodiment

In this embodiment, a temperature dependence on a transmittance-appliedvoltage characteristic of a light modulation apparatus was examined.

Temperature Dependence on Transmittance-applied Voltage Characteristicof Negative Type Liquid Crystal Device

A GH liquid crystal device 12 using a negative type nematic liquidcrystal, disposed at each of environmental temperatures (23.5° C., 40°C., 55° C., and 65° C.), was driven by applying a drive voltage havingan AC waveform (1 kHz) shown in FIG. 3C thereto, and atransmittance-applied voltage characteristic (V-T characteristic) of theGH liquid crystal device 12 was measured. The results are shown in FIG.26. At 23.5° C., the transmittance of the device is as bright as about80% in a range of 0 to 1.5 V, being reduced with an increase in appliedvoltage in a range of 2 V or more, and is gradually saturated in a rangeof 5 V or more.

Such a V-T characteristic has a temperature dependence shown in FIG. 26,in which the transmittance is decreased with an increase inenvironmental temperature in a range of 0 to about 4 V, and is raisedwith an increase in environmental temperature in a range of more thanabout 4 V. The change in transmittance caused by temperature changeoccurs due to thermal fluctuation of molecules of the liquid crystal asthe host-material and molecules of the pigment as the guest material ofthe GH liquid crystal device 12. To be more specific, if the moleculesof the liquid crystal and pigment are aligned in the directionperpendicular to a substrate plane, the components, projected on thesubstrate plane, of the absorption axes of the pigment molecules (inparallel to the major axes of the pigment molecules) are increased dueto the fluctuation of the molecules, with a result that the absorptionof light by the pigment molecules is increased, that is, the lightshielding characteristic is raised. On the contrary, if the molecules ofthe liquid crystal and pigment are aligned in the direction parallel tothe substrate plane, the components, projected on the substrate plane,of the absorption axes of the pigment molecules are decreased due to thefluctuation of the molecules, with a result that the absorption of lightby the pigment molecules is decreased, that is, the light shieldingcharacteristic is reduced;

Feedback Control Based on Monitored Light Detection Signal

To avoid a variation in transmittance depending on an environmentaltemperature, according to this embodiment, there was adopted a method ofmonitoring the controlled quantity of transmission light (that is, thecontrolled transmittance), comparing it with a setting transmittancepredetermined on the basis of an environmental temperature, and feedingback correction information to a control unit, thereby correcting thewaveform of the voltage applied to the liquid crystal device so as tomake constant the transmittance. With this method, the effect of anenvironmental temperature exerted on the transmittance can be eliminatedwithout directly monitoring the environmental temperature. For example,if an actual transmittance becomes larger than the setting transmittanceby the effect of temperature rise, the pulse height of each drive pulsemay be increased so that the actual transmittance corresponds to thesetting transmittance (see FIG. 26). The means for monitoring light maybe configured as a detector such as a photodiode, or an image pickupdevice, typically, a CCD (charge Coupled Device)

Control Method Based on Temperature Detection Signal

As shown in FIG. 26, the transmittance of the liquid crystal devicedriven by the same applied voltage differs depending on temperaturechange. The repeatability of the V-T characteristic of the liquidcrystal device, varied depending on an environmental temperature,however, is desirable. For example, as shown in FIG. 27, there can beobtained a substantially linear conversion relationship between avoltage applied to the liquid crystal for obtaining a transmittance at23.5° C. and a voltage applied to the liquid crystal device at 65° C.for obtaining the same transmittance.

Accordingly, a transmittance of the liquid crystal device can be usuallykept constant without effect of temperature change by monitoring anenvironmental temperature of the device, correcting the present voltageinto a new voltage corresponding to the monitored environmentaltemperature on the basis of the conversion relationship shown in FIG.27, or adding or subtracting a voltage difference (shown in FIG. 28)corresponding to a voltage at the monitored environmental temperature(shown in FIG. 27), which voltage difference is read out from a look-uptable, to or from the present voltage. In this way, according to thepresent invention, the transmittance characteristic of the liquidcrystal device less affected by an environmental temperature can beobtained by controlling the present pulse voltage into a voltagecorresponding to the monitored environmental temperature.

Temperature Dependence on Transmittance-applied Voltage Characteristicof Positive Type Liquid Crystal Device

The same GH cell as that described above except that a positive typeliquid crystal (trade name: MLC-6849, produced by Merck) was used as thehost material was prepared, and the temperature dependence on thetransmittance-applied voltage characteristic of the GH cell wasexamined.

As the result of the drive of the GH liquid crystal cell using such apositive type nematic liquid crystal by using an applied voltage havingan AC waveform (1 kHz) shown in FIG. 3C, it was found, as in the case ofFIG. 2, that on the low voltage side, the light absorption by the liquidcrystal was increased because of alignment of liquid crystal moleculesin the horizontal direction, and as shown in FIGS. 30 and 31, such lightabsorption was decreased with a temperature rise, and that on the highvoltage side, the light absorption by the liquid crystal was decreasedand as shown in FIGS. 30 and 31, such light absorption was increasedwith a temperature rise. Accordingly, even in this case, thetransmittance of the GH cell varied depending on the environmentaltemperature can be kept constant by the same manner as that describedabove.

Examples of positive type host materials (Δ∈>0) usable for the lightmodulation apparatus according to the present invention may includecompounds having the following molecular structures:

EXAMPLE 1

EXAMPLE 2

EXAMPLE 3

EXAMPLE 4

EXAMPLE 5

EXAMPLE 6

EXAMPLE 7

EXAMPLE 8

EXAMPLE 9

EXAMPLE 10

EXAMPLE 11

EXAMPLE 12

EXAMPLE 13

EXAMPLE 14

EXAMPLE 15

Examples of positive type host materials usable for the light modulationapparatus according to the present invention may include the followingcommercially available compounds:

[First Examples (trade names) of compounds produced by Merck] Phasetransition Δn Rotational temperature Index of V10 V90 Viscosity at 20°C. Viscosity at −30° C. viscosity (N→1) (° C.) birefringence (V) (V)(mm²/s) (mm²/s) at 20° C. (mPa · s) ZLI-4792 93.0 0.0969 2.14 3.21 15420 134 ZLI-5080 103.0 0.0864 1.91 2.89 21 870 220 ZLI-5091 99.5 0.10641.77 2.85 23 1200 220 MLC-6043-000 110.0 0.0894 1.76 2.78 24 1300 259MLC-6043-100 111.5 0.0997 1.82 2.80 25 1300 264 MLC-6219-000 98.0 0.08992.04 3.09 17 580 MLC-6219-100 97.5 0.1115 2.11 3.16 18 650 MLC-6222 98.50.0925 1.60 2.47 24 1220 MLC-6225-000 82.0 0.0966 2.10 3.17 17 630 119MLC-6225-100 83.0 0.1058 2.11 3.13 19 810 MLC-6241-000 100.0 0.0875 1.822.95 19 740 MLC-6241-100 100.0 0.0923 1.87 3.00 19 740 MLC-6252 98.00.0790 1.93 2.99 21 850 193 MLC-6256 98.5 0.1005 1.45 2.28 26 1640MLC-6292-000 120.0 0.0903 1.80 2.85 28 1450 MLC-6292-100 120.0 0.11461.83 2.83 25 1450 MLC-6625 83.5 0.0747 1.77 2.74 15 390 110 MLC-662888.0 0.0883 1.65 2.53 19 680 149 MLC-6694-000 112.5 0.0838 1.88 2.93 21920 194 MLC-6694-100 110.0 0.1060 1.88 2.86 21 1070 190 MLC-6846-00080.0 0.0897 1.30 2.01 172 MLC-6846-100 80.0 0.1083 1.27 1.95 195MLC-6847-000 90.5 0.0923 1.40 2.14 197 MLC-6847-100 90.5 0.1084 1.382.11 204 MLC-6848-000 70.5 0.0903 1.04 1.64 178 MLC-6848-100 70.5 0.10771.03 1.62 205 MLC-6849-000 91.0 0.0921 1.28 1.98 186 MLC-6849-100 90.00.1138 1.26 1.93 206 MLC-7700-000 98.0 0.0870 1.66 2.63 23 1000MLC-7700-100 100.0 0.1146 1.65 2.57 24 1300 205 MLC-7800-000 100.00.0854 2.11 3.21 19 700 MLC-7800-100 100.0 0.1149 2.13 3.24 19 870MLC-9000-000 88.0 0.0874 1.42 2.24 27 1500 244 MLC-9000-100 90.5 0.11371.41 2.22 30 2400 238 MLC-9100-000 91.0 0.0852 1.67 2.60 23 1030 193MLC-9100-100 89.0 0.1134 1.68 2.59 22 1100 166 MLC-9200-000 89.0 0.08482.13 3.22 18 590 MLC-9200-100 90.0 0.1146 2.18 3.24 19 880 MLC-9300-000110.0 0.0904 1.68 2.68 28 1500 284 MLC-9300-100 109.0 0.1154 1.71 2.6425 1550 237 MLC-9400-000 108.0 0.0892 2.25 3.44 20 780 188 MLC-9400-100110.0 0.1150 2.27 3.45 20 970 180 MLC-12000- 90.0 0.0876 1.42 2.22 221100 186 000 MLC-12000- 92.0 0.0860 1.68 2.65 18 700 148 100 MLC-12100-92.0 0.1128 1.47 2.22 24 1350 183 000 MLC-12100- 92.0 0.1105 1.74 2.5919 820 149 100 MLC-13200- 90.0 0.0871 1.48 2.30 23 1140 186 000MLC-13200- 94.5 0.0860 1.77 2.76 19 890 145 100 MLC-13300- 91.5 0.10931.47 2.29 24 1600 201 000 MLC-13300- 91.0 0.1078 1.77 2.68 20 870 155100 MLC-13800- 110.0 0.0902 1.69 2.61 228 000 MLC-13800- 111.0 0.09032.15 3.32 151 100 MLC-13900- 110.5 0.1070 1.63 2.50 235 000 MLC-13900-110.5 0.1081 2.15 3.27 167 100

Second Examples (Trade Names) of Compounds Produced by Chisso EXAMPLE 1

LIXON 5035XX S-N transition <−30.0° C. Cleaning temperature 82.2° C.Viscosity η at 20° C. 24.3 mPa · s at 0° C. 70.8 mPa · s at −20° C.287.9 mPa · s Resistivity ρ at 25° C. >1 × 10¹⁸ Ω − cm Opticalanisotropy Δn at 25° C. 589 nm 0.0749 n_(e) 1.5582 n_(o) 1.4833Dielectric Δε at 25° C. 1 kHz 4.4 anisotropy ε// 8.1 ε⊥ 3.7

EXAMPLE 2

LIXON 5036XX S-N transition <−30.0° C. Cleaning temperature 91.8° C.Viscosity η at 20° C. 26.0 mPa · s at 0° C. 79.3 mPa · s at −20° C.324.1 mPa · s Resistivity ρ at 25° C. >1 × 10¹⁸ Ω − cm Opticalanisotropy Δn at 25° C. 589 nm 0.0754 n_(e) 1.5586 n_(o) 1.4832Dielectric Δε at 25° C. 1 kHz 4.5 anisotropy ε// 8.1 ε⊥ 3.6

EXAMPLE 3

LIXON 5037XX S-N transition <−20.0° C. Cleaning temperature 101.4° C.Viscosity η at 20° C. 28.9 mPa · s at 0° C. 93.5 mPa · s at −20° C.370.6 mPa · s Resistivity ρ at 25° C. >1 × 10¹⁹ Ω − cm Opticalanisotropy Δn at 25° C. 589 nm 0.0752 n_(e) 1.5584 n_(o) 1.4832Dielectric Δε at 25° C. 1 kHz 4.5 anisotropy ε// 8.1 ε⊥ 3.6

EXAMPLE 4

LIXON 5038XX S-N transition <−30.0° C. Cleaning temperature 81.6° C.Viscosity η at 20° C. 25.0 mPa · s at 0° C. 71.1 mPa · s at −20° C.291.0 mPa · s Resistivity ρ at 25° C. >1 × 10¹³ Ω − cm Opticalanisotropy Δn at 25° C. 589 nm 0.0813 n_(e) 1.5671 n_(o) 1.4858Dielectric Δε at 25° C. 1 kHz 4.6 anisotropy ε// 8.3 ε⊥ 3.7

EXAMPLE 5

LIXON 5039XX S-N transition <−30.0° C. Cleaning temperature 91.1° C.Viscosity η at 20° C. 25.2 mPa · s at 0° C. 77.6 mPa · s at −20° C.317.2 mPa · s Resistivity ρ at 25° C. >1 × 10¹³ Ω − cm Opticalanisotropy Δn at 25° C. 589 nm 0.0806 n_(e) 1.5658 n_(o) 1.4852Dielectric Δε at 25° C. 1 kHz 4.7 anisotropy ε// 8.4 ε⊥ 3.7

EXAMPLE 6

LIXON 5040XX S-N transition <−30.0° C. Cleaning temperature 101.8° C.Viscosity η at 20° C. 28.4 mPa · s at 0° C. 93.5 mPa · s at −20° C.363.0 mPa · s Resistivity ρ at 25° C. >1 × 10¹⁸ Ω − cm Opticalanisotropy Δn at 25° C. 589 nm 0.0794 n_(e) 1.5649 n_(o) 1.4855Dielectric Δε at 25° C. 1 kHz 4.7 anisotropy ε// 8.3 ε⊥ 3.6

EXAMPLE 7

LIXON 5041XX S-N transition <−30.0° C. Cleaning temperature 81.7° C.Viscosity η at 20° C. 25.4 mPa · s at 0° C. 74.8 mPa · s at −20° C.302.0 mPa · s Resistivity ρ at 25° C. >1 × 10¹² Ω-cm Optical anisotropyΔn at 25° C., 589 nm 0.0847 n_(e) 1.5715 n_(o) 1.4868 Dielectric Δε at25° C., 1 kHz 4.7 anisotropy ε_(″) 8.4 ε⊥ 3.7

EXAMPLE 8

LIXON 5043XX S-N transition <−30.0° C. Cleaning temperature 101.9° C.Viscosity η at 20° C. 28.7 mPa · s at 0° C. 92.5 mPa · s at −20° C.354.6 mPa · s Resistivity ρ at 25° C. >1 × 10¹² Ω-cm Optical anisotropyΔn at 25° C., 589 nm 0.0850 n_(e) 1.5713 n_(o) 1.4863 Dielectric Δε at25° C., 1 kHz 4.9 anisotropy ε_(″) 8.5 ε⊥ 3.6

EXAMPLE 9

LIXON 5044XX S-N transition <−30.0° C. Cleaning temperature 81.0° C.Viscosity η at 20° C. 24.4 mPa · s at 0° C. 71.3 mPa · s at −20° C.293.1 mPa · s Resistivity ρ at 25° C. >1 × 10¹² Ω-cm Optical anisotropyΔn at 25° C., 589 nm 0.0895 n_(e) 1.5784 n_(o) 1.4889 Dielectric Δε at25° C., 1 kHz 4.9 anisotropy ε_(″) 8.7 ε⊥ 3.8

EXAMPLE 10

LIXON 5046XX S-N transition <−30.0° C. Cleaning temperature 100.3° C.Viscosity η at 20° C. 30.2 mPa · s at 0° C. 92.8 mPa · s at −20° C.372.9 mPa · s Resistivity ρ at 25° C. >1 × 10¹² Ω-cm Optical anisotropyΔn at 25° C., 589 nm 0.0895 n_(e) 1.5776 n_(o) 1.4881 Dielectric Δε at25° C., 1 kHz 4.9 anisotropy ε_(″) 8.6 ε⊥ 3.7

EXAMPLE 11

LIXON 5047XX S-N transition <−30.0° C. Cleaning temperature 80.3° C.Viscosity η at 20° C. 25.0 mPa · s at 0° C. 74.0 mPa · s at −20° C.306.8 mPa · s Resistivity ρ at 25° C. >1 × 10¹² Ω-cm Optical anisotropyΔn at 25° C., 589 nm 0.0997 n_(e) 1.5922 n_(o) 1.4925 Dielectric Δε at25° C., 1 kHz 5.1 anisotropy ε_(″) 8.9 ε⊥ 3.8

EXAMPLE 12

LIXON 5049XX S-N transition <−30.0° C. Cleaning temperature 101.0° C.Viscosity η at 20° C. 30.4 mPa · s at 0° C. 92.8 mPa · s at −20° C.429.3 mPa · s Resistivity ρ at 25° C. >1 × 10¹² Ω-cm Optical anisotropyΔn at 25° C., 589 nm 0. 1015 n_(e) 1.5935 n_(o) 1.4920 Dielectric Δε at25° C., 1 kHz 5.1 anisotropy ε_(″) 8.8 ε⊥ 3.7

EXAMPLE 13

LIXON 5050XX S-N transition <−30.0° C. Cleaning temperature 100.2° C.Viscosity η at 20° C. 23.5 mPa · s at 0° C. 69.1 mPa · s at −20° C.291.3 mPa · s Resistivity ρ at 25° C. >1 × 10¹² Ω-cm Optical anisotropyΔn at 25° C., 589 nm 0.0855 n_(e) 1.5732 n_(o) 1.4877 Dielectric Δε at25° C., 1 kHz 3.1 anisotropy ε_(″) 6.5 ε⊥ 3.4

EXAMPLE 14

LIXON 5051XX S-N transition <−20.0° C. Cleaning temperature 101.5° C.Viscosity η at 20° C. 23.9 mPa · s at 0° C. 69.1 mPa · s at −20° C.295.1 mPa · s Resistivity ρ at 25° C. >1 × 10¹² Ω-cm Optical anisotropyΔn at 25° C., 589 nm 0.0803 n_(e) 1.5794 n_(o) 1.4891 Dielectric Δε at25° C., 1 kHz 3.1 anisotropy ε_(″) 6.5 ε⊥ 3.4Control Method Based on Temperature Detection Signal

As shown in FIGS. 30 and 31, the transmittance of the positive typeliquid crystal device driven by the same applied voltage differsdepending on temperature change. The repeatability of the V-Tcharacteristic of the liquid crystal device, varied depending on anenvironmental temperature, however, is desirable. For example, as shownin FIG. 32, there can be obtained a substantially linear conversionrelationship between a voltage applied to the liquid crystal forobtaining a transmittance at 25° C. and a voltage applied to the liquidcrystal device at 65° C. for obtaining the same transmittance.

Accordingly, a transmittance of the liquid crystal device can be usuallykept constant without effect of temperature change by monitoring anenvironmental temperature of the device, correcting the present voltageinto a new voltage corresponding to the monitored environmentaltemperature on the basis of the conversion relationship shown in FIG.32, or adding or subtracting a voltage difference (shown in FIG. 29)corresponding to a voltage at the monitored environmental temperature(shown in FIG. 32), which voltage difference is read out from a look-uptable, to or from the present voltage. In this way, according to thepresent invention, the transmittance characteristic of the liquidcrystal device less affected by an environmental temperature can beobtained by controlling the present pulse voltage into a voltagecorresponding to the monitored environmental temperature.

Sixth Embodiment

One configuration example of a light modulation apparatus using the GHcell shown in FIGS. 3A to 3C will be described with reference to FIGS.33, 34, and 35A to 35C.

Referring to FIG. 33, there is shown a light modulation apparatus 23including a GH cell 12 and a polarizing plate 11. The GH cell 12, whichis enclosed between two glass substrates (not shown), contains negativetype liquid crystal molecules as a host material, and positive ornegative type dichroic dye molecules as a guest material. The negativetype liquid crystal molecules have a negative type dielectric constantanisotropy, and the dichroic dye molecules have a positive type lightabsorption anisotropy capable of absorbing light in the alignmentdirection of major axes of the molecules. The light absorption axis ofthe polarizing plate 11 is set in such a manner as to be madeperpendicular to the light absorption axis of the GH cell when a voltageis applied to the GH cell.

The light modulation apparatus 23 is disposed between a front lens group15 and a rear lens group 16 each of which is composed of a plurality oflenses such as zoom lenses. Light which has passed through the frontlens group 15 is linearly polarized via the polarizing plate 11 and ismade incident on the GH cell 12. The light emerged from the GH cell 12is collected by the rear lens group 16 and is projected as an image onan image pickup screen 17.

The polarizing plate 11 constituting part of the light modulationapparatus 23 is movable in or from an effective optical path of lightmade incident on the GH cell 12. To be more specific; the polarizingplate 11 can be moved to a position shown by a virtual line, to be thusmoved out of the effective path of light. A mechanical iris shown inFIG. 34 may be used as the means for moving the polarizing plate 11 inor from the effective optical path of light.

The mechanical iris , which is a mechanical diaphragm device generallyused for a digital still camera or a video camera, mainly includes twoiris blades 18 and 19, and a polarizing plate 11 stuck on the iris blade18. The iris blades 18 and 19 are movable in the vertical direction. Asshown in FIG. 34, the iris blades 18 and 19 are relatively moved in thedirections shown by arrows 21 by a drive motor (not shown).

With the relative movement of the iris blades 18 and 19, the iris blades18 and 19 are partially overlapped to each other as shown in FIG. 34,and as the overlapped area becomes larger, an opening 22 on an effectiveoptical path 20 positioned near a center portion between the iris blades18 and 19 comes to be covered with the polarizing plate 11.

FIG. 35A to 35C are partial enlarged views of a portion, near theeffective optical path 20, of the mechanical iris. When the iris blade18 is moved down and simultaneously the iris blade 19 is moved up; thepolarizing plate 11 stuck on the iris blade 18 is also moved out of theeffective optical path 20 as shown in FIG. 35A. On the contrary, whenthe iris blade 18 is moved up and simultaneously the iris blade 19 ismoved down, the iris blades 18 and 19 are overlapped to each other, andas shown in FIG. 35B, the polarizing plate 11 is moved on the effectiveoptical path 20, to gradually cover the opening 22. As shown in FIG.35C, the overlapped area of the iris blades 18 and 19 becomes larger,the polarizing plate 11 comes to perfectly cover the opening 20.

The operation of the light modulation apparatus 23 using the mechanicaliris will be described below.

As an object (not shown) becomes bright, the iris blades 18 and 19,which are opened in the upward and downward directions as shown in FIG.35A, are driven by the motor (not shown) to be gradually overlapped toeach other. Along with such movement of the iris blades 18 and 19, thepolarizing plate 11 stuck on the iris blade 18 starts to enter theeffective optical path 20, thereby partially covering the opening 22 asshown in FIG. 35B.

At this time, the GH cell 12 is in a state not allowed to absorb lightexcept for slight light absorption due to thermal fluctuation or surfacereflection. Accordingly, the intensity distribution of the light havingpassed through the polarizing plate 11 is nearly equal to that of thelight having passed through the opening 22.

The polarizing plate 11 is then put in a state in which it perfectlycovers the opening 22 as shown in FIG. 35C. In such a state if thebrightness of the object becomes stronger, the voltage applied to the GHcell 12 is increased to modulate the light by absorbing the light in theGH cell 12.

On the contrary, in the above state, if the object becomes dark, thevoltage applied to the GH cell 12 is reduced or cut off to eliminate thelight absorption effect by the GH cell 12. If the object becomes darker,the iris blade 18 is moved down and the iris blade 19 is moved up by themotor (not shown), to move the polarizing plate 11 out of the effectiveoptical path 20 as shown in FIG. 35A.

According to this embodiment, since the polarizing plate 11 whosetransmittance is typically in a range of 40 to 50% can be moved out ofthe effective optical path 20 of light, the light is not absorbed in thepolarizing plate 11, with a result that the maximum transmittance of thelight modulation apparatus of the present invention can be increased upto a value being as high as twice or more the maximum transmittance ofthe related art light modulation apparatus including the GH cell and thefixed polarizing plate. It should be noted that the minimumtransmittance of the light modulation apparatus of the present inventionis equal to that of the related art light modulation apparatus.

Since the polarizing plate 11 is moved in or from the effective opticalpath of light by using the mechanical iris practically used for adigital still camera, the light modulation apparatus having the aboveconfiguration can be easily realized.

Since the light modulation apparatus in this embodiment uses the GH cell12, the light modulation can be effectively performed by the combinationof the light modulation by the polarizing plate 11 and light absorptionof the GH cell 12.

In this way, according to the light modulation apparatus in thisembodiment, it is possible to enhance the bright-dark contrast ratio andto keep the light quantity distribution at a nearly constant value.

With respect to the GH cell 12 used in this embodiment, if the lightcrystal molecules having a negative dielectric constant anisotropy isused as the host material, a negative type (n-type) dichroic dyemolecules may be used as the guest material.

The related art light modulation apparatus shown in FIGS. 1A to 1C has aproblem. Since the polarizing plate 1 is fixed in an effective opticalpath of light, part of light, for example, 50% of light is usuallyabsorbed in the polarizing plate 1, and further light may be reflectedfrom the surface of the polarizing plate 1. As a result, the maximumtransmittance of light passing through the polarizing plate 1 cannotexceed a certain value, for example, 50%, and accordingly, the quantityof light passing through the light modulation apparatus is significantlyreduced by light absorption of the polarizing plate 1. This problem isone of the factors which make it difficult to put a light modulationapparatus using a liquid crystal cell into practical use.

On the other hand, various kinds of light modulation apparatuses usingno polarizing plate have been proposed. Examples of these apparatusesinclude a type using a stack of two GH cells in which the GH cell at thefirst layer absorbs a polarization component in the direction identicalto that of polarized light and the GH cell at the second layer absorbs apolarization component in the direction perpendicular to the polarizedlight; a type making use of a phase transition between a cholestericphase and a nematic phase of a liquid crystal cell; and a high polymerscattering type making use of scattering of liquid crystal.

These light modulation apparatuses using no polarizing plate have aproblem. Since the optical density (absorbance) ratio between uponapplication of no voltage and upon application of a voltage is, asdescribed above, as small as only 5, the contrast ratio of the apparatusis too small to normally carry out modulation of light at any locationin a wide range from a bright location to a dark location. The lightmodulation apparatus of the high polymer scattering type has anotherproblem in significantly degrading, when the apparatus is used for animage pickup apparatus, the image formation performance of an opticalsystem of the image pickup apparatus.

The related art light modulation apparatus presents a further problem.Since the transmittance in a transparent state may become dark dependingon the kind of a liquid crystal device used for the apparatus, if animage pickup apparatus provided with the light modulation apparatus isintended to pickup image with a sufficient light quantity in such atransparent state, the light modulation apparatus is required to beremoved from an optical system of the image pickup apparatus.

On the contrary, according to this embodiment, since the polarizingplate 11 is movable in or from the effective optical path of light, itis possible to increase the quantity of light, enhance the contrastratio, and keep constant the quantity of light.

Seventh Embodiment

One example in which the light modulation apparatus 23 described in thesixth embodiment is assembled in a CCD (Charge Coupled Device) camerawill be described with reference to FIGS. 36 to 38.

Referring to FIG. 36, there is shown a CCD camera 50 including a firstlens group 51 and a second lens group (for zooming) 52, which areequivalent to the above-described front lens group 15; a third lensgroup 53 and a fourth lens group (for focusing) 54, which are equivalentto the above-described rear lens group 16; and a CCD package 55. Thesecomponents of the CCD camera 50 are spaced in this order at suitableintervals along an optical axis shown by a dashed line. An infrared raycutoff filter 55 a, an optical low pass filter system 55 b, and a CCDimage pickup device 55 c are contained in the CCD package 55. The lightmodulation apparatus 23 including the GH cell 12 and the polarizingplate 11 according to the present invention for adjusting or restrictingthe quantity of light is mounted on the same optical path at a positionwhich is located between the second lens group 52 and the third lensgroup 53 while being offset toward the third lens group 53. The fourthlens group 54 for focussing is movable along the optical path in a rangebetween the third lens group 53 and the CCD package 55 by a linear motor57, and the second lens group 52 for zooming is movable along theoptical path in a range between the first lens group 51 and the lightmodulation apparatus 23.

FIG. 37 shows an algorithm of a control sequence of a transmittance ofthe cameral system by the light modulation apparatus 23.

According to this embodiment, since the light modulation apparatus 23 ofthe present invention set between the second lens group 52 and the thirdlens group 53 can adjust the quantity of light by an electric fieldapplied thereto, it is possible to miniaturize the system andsubstantially reduce an effective range of an optical path, and hence tominiaturize the CCD camera. Since the quantity of light can be suitablycontrolled by the value of a voltage applied to patterned electrodes, itis possible to prevent a conventional diffraction phenomenon and toeliminate the dimming of an image by making a sufficient quantity oflight incident on the image pickup device.

Drive Circuit of Camera System

FIG. 38 is a block diagram showing a drive circuit of the CCD camera.Referring to FIG. 38, the CCD camera includes a drive circuit unit 60 ofthe CCD image pickup device 55 c disposed in the light outgoing side ofthe light modulation apparatus 23. An output signal from the CCD imagepickup device 55 c is processed by an Y/C signal processing unit 61 andis fed back as luminance information (Y signal) to a GH cell drivecontrol circuit unit 62. An environmental temperature of the GH cell 12is detected by a thermistor 65 and the detection temperature informationis fed back to the control circuit unit 62. Each drive pulse, whosepulse height or pulse width is modulated in synchronization with a basicclock generated from the drive circuit unit 60 on the basis of a controlsignal from the control circuit unit 62, is generated from a pulsegeneration circuit unit 63. The control circuit unit 62 and the pulsegeneration circuit unit 63 constitute a GH liquid crystal drive circuitunit 64 for modulating the pulse height or pulse width of each drivepulse.

The above-described camera system may be replaced with another system inwhich light emerged from the light modulation apparatus 23 is detectedby a photodetector (or photomultiplier tube); luminance information ofthe light detected by the photodetector is fed back, together withtemperature information detected by the thermistor 65, to the controlcircuit unit 62; and each drive pulse, whose pulse height or pulse widthis modulated in synchronization with a clock generated by a GH celldrive circuit unit (not shown) on the basis of the above luminanceinformation and temperature information, is generated by the pulsegeneration circuit unit.

Additionally, a basic clock such as a field decision signal, a verticalsynchronization signal, or a blacking signal, or a reset gate signal canbe used as a synchronization signal for changing the GH control waveformin a period other than a CCD image pickup accumulation time. Further,the luminance information and temperature information can beindependently or simultaneously fed back.

Although the preferred embodiments of the present invention have beendescribed, such description is for illustrative purposes only, and it isto be understood that many changes and variations may be made withoutdeparting from the technical thought of the present invention.

For example, the structure and material of each of the liquid crystaldevice and polarizing plate, its drive mechanism, and the configurationof each of the drive circuit and control circuit may be variouslychanged. The drive waveform of each drive pulse applied betweenelectrodes may be a rectangular, trapezoidal or sine waveform insofar asit allows changes in directors of liquid crystal molecules, to controlthe transmittance of the light modulation apparatus. The means fordetecting the temperature of the light modulation apparatus is notlimited to the thermistor but may be another sensor.

The GH cell is not limited to that described in the embodiments but maybe a GH cell having a double-layer structure. Although the position ofthe polarizing plate 11 relative to the GH cell 12 is located betweenthe front lens group 15 and the rear lens group 16 in the embodiment, itis not limited thereto but may be suitably determined in considerationof the setting conditions of the image pickup lenses. To be morespecific, the polarizing plate 11 may be freely located on the objectside or image pickup device side, for example, at a position between theimage pickup screen 17 and the rear lens group 16. Further, thepolarizing plate 11 may be disposed in front of or at the back of asingle lens changed from the front lens group 15 or rear lens group 16.

The number of the iris blades 18 and 19 is not limited to two, but maybe one or two or more. The iris blades 18 and 19 may be moved in thedirection other than the vertical direction to be overlapped to eachother, or may be spirally moved in the direction from the periphery tothe center.

While the polarizing plate 11 is stuck on the iris blade 18 in theembodiment, it may be stuck on the iris blade 19.

In the embodiment, as the object becomes bright, the light modulation bymovement of the polarizing plate 11 is first performed and then thelight absorption by the GH cell 12 is performed; however, as the objectbecomes bright, the light absorption by the GH cell 12 may be firstperformed until the transmittance of the GH cell 12 is reduced to aspecific value and then the light modulation by movement of thepolarizing plate 11 be performed.

The means of moving the polarizing plate 11 in or from the effectiveoptical path 20 is not limited to the mechanical iris. For example, themovement of the polarizing plate 11 in or from the effective opticalpath 20 may be performed by directly providing a film, on which thepolarizing plate 11 is stuck, on a drive motor and operating the drivemotor.

In the embodiment, the polarizing plate 11 is moved in or from theeffective optical path 20; however, it may be of course fixed on theeffective optical path.

The light modulation apparatus may be used in combination with any oneof known filter materials such as an organic electrochromic material,liquid crystal, or electroluminescence material or the like.

The light modulation apparatus of the present invention may be used notonly for an optical diaphragm of an image pickup apparatus such as a CCDcamera but also for other optical systems such as a light quantityadjustment device for an electrophotographic reproduction machine oroptical communication equipment. The light modulation apparatus may beused not only for an optical filter but also for other image displaydevices for displaying characters or images.

1. A light modulation apparatus comprising: a liquid crystal device; apulse control unit for changing the transmittance of light made incidenton said liquid crystal device from a current transmittance into a targettransmittance by sequentially applying at least two distinct drivepulses to said liquid crystal device; and a polarizing plate that ismovable into and out of an optical path of light made incident on saidliquid crystal device; wherein said at least two drive pulses include afirst drive pulse having a first pulse height and a first pulse widthand a second drive pulse having a second pulse height and a second pulsewidth; and wherein the first pulse height is greater than the secondpulse height and/or the first pulse width is greater than the secondpulse width.
 2. A light modulation apparatus according to claim 1,further comprising a drive circuit unit, wherein the drive pulses aregenerated in synchronization with a clock generated by said drivecircuit unit.
 3. A light modulation apparatus according to claim 2,further comprising a control circuit unit, wherein luminance informationof the light emerged from said liquid crystal device is fed back to saidcontrol circuit unit, and the drive pulses are generated insynchronization with said clock, which is generated by said drivecircuit unit on the basis of a control signal supplied from said controlcircuit unit.
 4. A light modulation apparatus according to claim 1,wherein said polarizing plate is disposed in a movable portion of amechanical iris in such a manner as to be movable into and out of theoptical path by operation of said movable portion of said mechanicaliris.
 5. A light modulation apparatus according to claim 1, wherein adrive electrode of said liquid crystal device is formed at least overthe entire region of an effective light transmission portion.
 6. Animage pickup apparatus comprising: a light modulation apparatusincluding a liquid crystal device and a pulse control unit for changingthe transmittance of light made incident on said liquid crystal devicefrom a current transmittance into a target transmittance by sequentiallyapplying at least two distinct drive pulses to said liquid crystaldevice; wherein said light modulation apparatus is disposed in anoptical path of an optical system of said image pickup apparatus;wherein said at least two drive pulses include a first drive pulsehaving a first pulse height and a first pulse width and a second drivepulse having a second pulse height and a second pulse width; and whereinthe first pulse height is greater than the second pulse height and/orthe first pulse width is greater than the second pulse width.
 7. Animage pickup apparatus according to claim 6, further comprising a drivecircuit unit, wherein the drive pulses are generated in synchronizationwith a clock generated by said drive circuit unit.
 8. An image pickupapparatus according to claim 7, wherein said drive circuit unit is adrive circuit unit of an image pickup device disposed on a lightoutgoing side of said light modulation apparatus, and luminanceinformation of the light emerged from said liquid crystal device is fedback to said control circuit unit, and the drive pulses are generated insynchronization with said clock, which is generated by said drivecircuit unit on the basis of a control signal supplied from said controlcircuit unit.
 9. An image pickup apparatus according to claim 6, whereinsaid liquid crystal device is a guest-host type liquid crystal device.10. An image pickup apparatus according to claim 9, wherein a hostmaterial of said liquid crystal device is a negative or positive typeliquid crystal having a negative or positive type dielectric constantanisotropy.
 11. An image pickup apparatus according to claim 9, whereina guest material of said liquid crystal device is a positive or negativetype dichroic dye molecular material having a positive or negative typelight absorption anisotropy.
 12. An image pickup apparatus according toclaim 6, further comprising a polarizing plate disposed in an opticalpath of light made incident on said liquid crystal device.
 13. An imagepickup apparatus according to claim 6, further comprising a polarizingplate that is movable into and out of an optical path of light madeincident on said liquid crystal device.
 14. An image pickup apparatusaccording to claim 13, wherein said polarizing plate is disposed in amovable portion of a mechanical iris in such a manner as to be movableinto and out of the optical path by operation of said movable portion ofsaid mechanical iris.
 15. An image pickup apparatus according to claim6, wherein a drive electrode of said liquid crystal device is formed atleast over the entire region of an effective light transmission portion.16. A method of driving a light modulation apparatus including a liquidcrystal device, comprising the step of: changing the transmittance oflight made incident on said liquid crystal device from a currenttransmittance into a target transmittance by sequentially applying atleast two distinct drive pulses to said liquid crystal device; whereinthe light modulation apparatus includes a polarizing plate that ismovable into and out of an optical path of light made incident on saidliquid crystal device; wherein the at least two drive pulses include afirst drive pulse having a first pulse height and a first pulse widthand a second drive pulse having a second pulse height and a second pulsewidth; and wherein the first pulse height is greater than the secondpulse height and/or the first pulse width is greater than the secondpulse width.
 17. A method of driving a light modulation apparatusaccording to claim 16, wherein the drive pulses are generated insynchronization with a clock generated by a drive circuit unit providedin said light modulation apparatus.
 18. A method of driving a lightmodulation apparatus according to claim 17, wherein luminanceinformation of the light emerged from said liquid crystal device is fedback to a control circuit unit provided in said light modulationapparatus, and the drive pulses are generated in synchronization withthe clock, which is generated by said drive circuit unit on the basis ofa control signal supplied from said control circuit unit.
 19. A methodof driving a light modulation apparatus according to claim 16, furthercomprising selectively moving said polarizing plate into and out of saidoptical path of light made incident on said liquid crystal device.
 20. Amethod of driving a light modulation apparatus according to claim 19,wherein said polarizing plate is disposed in a movable portion of amechanical iris in such a manner as to be movable into and out of theoptical path by operation of said movable portion of said mechanicaliris.
 21. A method of driving a light modulation apparatus according toclaim 16, wherein a drive electrode of said liquid crystal device isformed at least over the entire region of an effective lighttransmission portion.
 22. A method of driving an image pickup apparatusin which a liquid crystal device of a light modulation apparatus isdisposed in an optical path of an optical system of said image pickupapparatus, comprising the step of: changing the transmittance of lightmade incident on said liquid crystal device from a current transmittanceinto a target transmittance by sequentially applying at least twodistinct drive pulses to said liquid crystal device; wherein the atleast two drive pulses include a first drive pulse having a first pulseheight and a first pulse width and a second drive pulse having a secondpulse height and a second pulse width; and wherein the first pulseheight is greater than the second pulse height and/or the first pulsewidth is greater than the second pulse width.
 23. A method of driving animage pickup apparatus according to claim 22, wherein the drive pulsesare generated in synchronization with a clock generated by a drivecircuit unit provided in said light modulation apparatus.
 24. A methodof driving an image pickup apparatus according to claim 22, wherein adrive circuit unit is disposed on a light outgoing side of said lightmodulation apparatus, luminance information of the light emerged fromsaid liquid crystal device is fed back to a control circuit unitprovided in said light modulation apparatus, and the drive pulses aregenerated in synchronization with a clock generated by said drivecircuit unit on the basis of a control signal supplied from said controlcircuit unit.
 25. A method of driving an image pickup apparatusaccording to claim 22, wherein said liquid crystal device is aguest-host type liquid crystal device.
 26. A method of driving an imagepickup apparatus according to claim 25, wherein a host material of saidliquid crystal device is a negative or positive type liquid crystalhaving a negative or positive type dielectric constant anisotropy.
 27. Amethod of driving an image pickup apparatus according to claim 25,wherein a guest material of said liquid crystal device is a positive ornegative type dichroic dye molecular material having a positive ornegative type light absorption anisotropy.
 28. A method of driving animage pickup apparatus according to claim 22, wherein a polarizing plateis disposed in an optical path of light made incident on said liquidcrystal device.
 29. A method of driving an image pickup apparatusaccording to claim 22, further comprising selectively moving apolarizing plate into and out of an optical path of light made incidenton said liquid crystal device.
 30. A method of driving an image pickupapparatus according to claim 29, wherein said polarizing plate isdisposed in a movable portion of a mechanical iris in such a manner asto be movable into and out of the optical path by operation of saidmovable portion of said mechanical iris.
 31. A method of driving animage pickup apparatus according to claim 22, wherein a drive electrodeof said liquid crystal device is formed at least over the entire regionof an effective light transmission portion.